When building bcachefs for 32-bit ARM, there is a compiler warning in
bch2_set_bucket_needs_journal_commit() due to a debug print using the
wrong specifier:

  fs/bcachefs/buckets_waiting_for_journal.c:137:30: error: format specifies type 'size_t' (aka 'unsigned int') but the argument has type 'unsigned long' [-Werror,-Wformat]
    136 |         pr_debug("took %zu rehashes, table at %zu/%zu elements",
        |                                                   ~~~
        |                                                   %lu
    137 |                  nr_rehashes, nr_elements, 1UL << b->t->bits);
        |                                            ^~~~~~~~~~~~~~~~~
  include/linux/printk.h:579:26: note: expanded from macro 'pr_debug'
    579 |         dynamic_pr_debug(fmt, ##__VA_ARGS__)
        |                          ~~~    ^~~~~~~~~~~
  include/linux/dynamic_debug.h:270:22: note: expanded from macro 'dynamic_pr_debug'
    270 |                            pr_fmt(fmt), ##__VA_ARGS__)
        |                                   ~~~     ^~~~~~~~~~~
  include/linux/dynamic_debug.h:250:59: note: expanded from macro '_dynamic_func_call'
    250 |         _dynamic_func_call_cls(_DPRINTK_CLASS_DFLT, fmt, func, ##__VA_ARGS__)
        |                                                                  ^~~~~~~~~~~
  include/linux/dynamic_debug.h:248:65: note: expanded from macro '_dynamic_func_call_cls'
    248 |         __dynamic_func_call_cls(__UNIQUE_ID(ddebug), cls, fmt, func, ##__VA_ARGS__)
        |                                                                        ^~~~~~~~~~~
  include/linux/dynamic_debug.h:224:15: note: expanded from macro '__dynamic_func_call_cls'
    224 |                 func(&id, ##__VA_ARGS__);                       \
        |                             ^~~~~~~~~~~
  1 error generated.

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

Fixes: 7a82e75ddaef ("bcachefs: New data structure for buckets waiting on journal commit")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>