When building for a 32-bit architecture, for which 'size_t' is
'unsigned int', there is a compiler warning due to use of '%lu':

  In file included from fs/bcachefs/vstructs.h:5,
                   from fs/bcachefs/bcachefs_format.h:80,
                   from fs/bcachefs/bcachefs.h:207,
                   from fs/bcachefs/btree_key_cache.c:3:
  fs/bcachefs/btree_key_cache.c: In function 'bch2_btree_key_cache_to_text':
  fs/bcachefs/btree_key_cache.c:795:25: error: format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
    795 |         prt_printf(out, "pending:\t%lu\r\n",            per_cpu_sum(bc->nr_pending));
        |                         ^~~~~~~~~~~~~~~~~~~
  fs/bcachefs/util.h:78:63: note: in definition of macro 'prt_printf'
     78 | #define prt_printf(_out, ...)           bch2_prt_printf(_out, __VA_ARGS__)
        |                                                               ^~~~~~~~~~~
  fs/bcachefs/btree_key_cache.c:795:38: note: format string is defined here
    795 |         prt_printf(out, "pending:\t%lu\r\n",            per_cpu_sum(bc->nr_pending));
        |                                    ~~^
        |                                      |
        |                                      long unsigned int
        |                                    %u
  cc1: all warnings being treated as errors

Use the proper specifier, '%zu', to resolve the warning.

Fixes: e447e49977b8 ("bcachefs: key cache can now allocate from pending")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>