bcachefs: Fix -Wformat in bch2_alloc_v4_invalid()

bcachefs: Fix -Wformat in bch2_alloc_v4_invalid()

When building bcachefs for 32-bit ARM, there is a compiler warning in
bch2_alloc_v4_invalid() due to use of an incorrect format specifier:

  fs/bcachefs/alloc_background.c:246:30: error: format specifies type 'unsigned long' but the argument has type 'unsigned int' [-Werror,-Wformat]
    245 |                 prt_printf(err, "bad val size (%u > %lu)",
        |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |                                                     %u
    246 |                        alloc_v4_u64s(a.v), bkey_val_u64s(k.k));
        |                        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
  fs/bcachefs/bkey.h:58:27: note: expanded from macro 'bkey_val_u64s'
     58 | #define bkey_val_u64s(_k)       ((_k)->u64s - BKEY_U64s)
        |                                 ^
  fs/bcachefs/util.h:223:54: note: expanded from macro 'prt_printf'
    223 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~

This expression is of type 'size_t'. 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: 11be8e8db283 ("bcachefs: New on disk format: Backpointers")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>