When building for 32-bit platforms, for which size_t is 'unsigned int',
there is a warning from a format string in validate_bset_keys():

  fs/bcachefs/btree_io.c: In function 'validate_bset_keys':
  fs/bcachefs/btree_io.c:891:34: error: format '%lu' expects argument of type 'long unsigned int', but argument 12 has type 'unsigned int' [-Werror=format=]
    891 |                                  "bad k->u64s %u (min %u max %lu)", k->u64s,
        |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  fs/bcachefs/btree_io.c:603:32: note: in definition of macro 'btree_err'
    603 |                                msg, ##__VA_ARGS__);                     \
        |                                ^~~
  fs/bcachefs/btree_io.c:887:21: note: in expansion of macro 'btree_err_on'
    887 |                 if (btree_err_on(!bkeyp_u64s_valid(&b->format, k),
        |                     ^~~~~~~~~~~~
  fs/bcachefs/btree_io.c:891:64: note: format string is defined here
    891 |                                  "bad k->u64s %u (min %u max %lu)", k->u64s,
        |                                                              ~~^
        |                                                                |
        |                                                                long unsigned int
        |                                                              %u
  cc1: all warnings being treated as errors

BKEY_U64s is size_t so the entire expression is promoted to size_t. Use
the '%zu' specifier so that there is no warning regardless of the width
of size_t.

Fixes: 031ad9e7 ("bcachefs: Check for packed bkeys that are too big")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404130747.wH6Dd23p-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202404131536.HdAMBOVc-lkp@intel.com/Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>