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Fake flexible arrays (zero-length and one-element arrays) are
deprecated, and should be replaced by flexible-array members.
So, replace zero-length array with a flexible-array member in
`struct bch_ioctl_fsck_offline`.

Also annotate array `devs` with `__counted_by()` to prepare for the
coming implementation by GCC and Clang of the `__counted_by` attribute.
Flexible array members annotated with `__counted_by` can have their
accesses bounds-checked at run-time via `CONFIG_UBSAN_BOUNDS` (for
array indexing) and `CONFIG_FORTIFY_SOURCE` (for strcpy/memcpy-family
functions).

This fixes the following -Warray-bounds warnings:
fs/bcachefs/chardev.c: In function 'bch2_ioctl_fsck_offline':
fs/bcachefs/chardev.c:363:34: warning: array subscript 0 is outside array bounds of '__u64[0]' {aka 'long long unsigned int[]'} [-Warray-bounds=]
  363 |         if (copy_from_user(devs, &user_arg->devs[0], sizeof(user_arg->devs[0]) * arg.nr_devs)) {...