bcachefs: Fix -Wformat in bch2_bucket_gens_invalid()

When building bcachefs for 32-bit ARM, there is a compiler warning in
bch2_bucket_gens_invalid() due to use of an incorrect format specifier:

  fs/bcachefs/alloc_background.c:530:10: error: format specifies type 'unsigned long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
    529 |                 prt_printf(err, "bad val size (%lu != %zu)",
        |                                                ~~~
        |                                                %zu
    530 |                        bkey_val_bytes(k.k), sizeof(struct bch_bucket_gens));
        |                        ^~~~~~~~~~~~~~~~~~~
  fs/bcachefs/util.h:223:54: note: expanded from macro 'prt_printf'
    223 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~

On 64-bit architectures, size_t is 'unsigned long', so there is no
warning when using %lu but on 32-bit architectures, size_t is 'unsigned
int'. Use '%zu', the format specifier for 'size_t', to eliminate the
warning.

Fixes: 4be0d766a7e9 ("bcachefs: bucket_gens btree")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>