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':
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>