bcachefs: byte order swap bch_alloc_v4.fragmentation_lru field

bcachefs: byte order swap bch_alloc_v4.fragmentation_lru field

A simple test to populate a filesystem on one CPU architecture and
fsck on an arch of the opposite byte order produces errors related
to the fragmentation LRU. This occurs because the 64-bit
fragmentation_lru field is not byte-order swapped when reads detect
that the on-disk/bset key values were written in opposite byte-order
of the current CPU.

Update the bch2_alloc_v4 swab callback to handle fragmentation_lru
as is done for other multi-byte fields. This doesn't affect existing
filesystems when accessed by CPUs of the same endianness because the
->swab() callback is only called when the bset flags indicate an
endianness mismatch between the CPU and on-disk data.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>