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

When building bcachefs for 32-bit ARM, there is a warning when using
min() to compare a variable of type 'size_t' with an expression of type
'unsigned long':

  fs/bcachefs/checksum.c:142:22: error: comparison of distinct pointer types ('typeof (len) *' (aka 'unsigned int *') and 'typeof (((1UL) << 12) - offset) *' (aka 'unsigned long *')) [-Werror,-Wcompare-distinct-pointer-types]
    142 |                         unsigned pg_len = min(len, PAGE_SIZE - offset);
        |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  include/linux/minmax.h:69:19: note: expanded from macro 'min'
     69 | #define min(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 min_t(size_t, ...) for
this situation, eliminating the warning.

Fixes: 1fb50457684f ("bcachefs: Fix memory corruption in encryption path")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>