XArray tests: Handle larger indices more elegantly

xa_mk_value() only handles values up to LONG_MAX.  I successfully hid
that inside xa_store_index() and xa_erase_index(), but it turned out I
also needed it for testing xa_alloc() on 32-bit machines.  So extract
xa_mk_index() from the above two functions, and convert the non-constant
users of xa_mk_value() to xa_mk_index().
Signed-off-by: Matthew Wilcox <willy@infradead.org>