bcachefs: Workaround for kvmalloc() not supporting > INT_MAX allocations

bcachefs: Workaround for kvmalloc() not supporting > INT_MAX allocations

kvmalloc() doesn't support allocations > INT_MAX, but vmalloc() does -
the limit should be lifted, but we can work around this for now.

A user with a 75 TB filesystem reported the following journal replay
error:
https://github.com/koverstreet/bcachefs/issues/769

In journal replay we have to sort and dedup all the keys from the
journal, which means we need a large contiguous allocation. Given that
the user has 128GB of ram, the 2GB limit on allocation size has become
far too small.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>