bcachefs: Fix -Wcompare-distinct-pointer-types in bch2_copygc_get_buckets()

bcachefs: Fix -Wcompare-distinct-pointer-types in bch2_copygc_get_buckets()

When building bcachefs for 32-bit ARM, there is a warning when using
max() to compare an expression involving 'size_t' with an 'unsigned
long' literal:

  fs/bcachefs/movinggc.c:159:21: error: comparison of distinct pointer types ('typeof (16UL) *' (aka 'unsigned long *') and 'typeof (buckets_in_flight->nr / 4) *' (aka 'unsigned int *')) [-Werror,-Wcompare-distinct-pointer-types]
    159 |         size_t nr_to_get = max(16UL, buckets_in_flight->nr / 4);
        |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  include/linux/minmax.h:76:19: note: expanded from macro 'max'
     76 | #define max(x, y)       __careful_cmp(x, y, >)
        |                         ^~~~~~~~~~~~~~~~~~~~~~
  include/linux/minmax.h:38:24: note: expanded from macro '__careful_cmp'
     38 |         __builtin_choose_expr(__safe_cmp(x, y), \
        |                               ^~~~~~~~~~~~~~~~
  include/linux/minmax.h:28:4: note: expanded from macro '__safe_cmp'
     28 |                 (__typecheck(x, y) && __no_side_effects(x, y))
        |                  ^~~~~~~~~~~~~~~~~
  include/linux/minmax.h:22:28: note: expanded from macro '__typecheck'
     22 |         (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
        |                    ~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~
  1 error generated.

On 64-bit architectures, size_t is 'unsigned long', so there is no
warning when comparing these two expressions. Use max_t(size_t, ...) for
this situation, eliminating the warning.

Fixes: dd49018737d4 ("bcachefs: Rhashtable based buckets_in_flight for copygc")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>