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>

fs/bcachefs/alloc_background.c

@@ -319,6 +319,7 @@ void bch2_alloc_v4_swab(struct bkey_s k)
 	a->io_time[1]		= swab64(a->io_time[1]);
 	a->stripe		= swab32(a->stripe);
 	a->nr_external_backpointers = swab32(a->nr_external_backpointers);
+	a->fragmentation_lru	= swab64(a->fragmentation_lru);
 
 	bps = alloc_v4_backpointers(a);
 	for (bp = bps; bp < bps + BCH_ALLOC_V4_NR_BACKPOINTERS(a); bp++) {