When building for a 32-bit target, for which 'size_t' is 'unsigned int',
there are two warnings around mismatched format specifiers and argument
types:

  In file included from fs/bcachefs/vstructs.h:5,
                   from fs/bcachefs/bcachefs_format.h:79,
                   from fs/bcachefs/bcachefs.h:207,
                   from fs/bcachefs/btree_key_cache.c:3:
  fs/bcachefs/btree_key_cache.c: In function 'bch2_btree_key_cache_to_text':
  fs/bcachefs/btree_key_cache.c:1046:25: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
   1046 |         prt_printf(out, "nonpcpu freelist:\t%lu\r\n",   bc->nr_freed_nonpcpu);
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~
        |                                                           |
        |                                                           size_t {aka unsigned int}
  fs/bcachefs/util.h:192:63: note: in definition of macro 'prt_printf'
    192 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~
  fs/bcachefs/btree_key_cache.c:1046:47: note: format string is defined here
   1046 |         prt_printf(out, "nonpcpu freelist:\t%lu\r\n",   bc->nr_freed_nonpcpu);
        |                                             ~~^
        |                                               |
        |                                               long unsigned int
        |                                             %u
  fs/bcachefs/btree_key_cache.c:1047:25: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
   1047 |         prt_printf(out, "pcpu freelist:\t%lu\r\n",      bc->nr_freed_pcpu);
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~       ~~~~~~~~~~~~~~~~~
        |                                                           |
        |                                                           size_t {aka unsigned int}
  fs/bcachefs/util.h:192:63: note: in definition of macro 'prt_printf'
    192 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~
  fs/bcachefs/btree_key_cache.c:1047:44: note: format string is defined here
   1047 |         prt_printf(out, "pcpu freelist:\t%lu\r\n",      bc->nr_freed_pcpu);
        |                                          ~~^
        |                                            |
        |                                            long unsigned int
        |                                          %u
  cc1: all warnings being treated as error

Use the proper 'size_t' specifier, '%zu', to clear up the warnings for
these platforms.

Fixes: f2d47ec26af5 ("bcachefs: Btree key cache instrumentation")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>