xfs: make staging file forks explicit

xfs: make staging file forks explicit

Source kernel commit: 42e357c806c8c0ffb9c5c2faa4ad034bfe950d77

Don't open-code "-1" for whichfork when we're creating a staging btree
for a repair; let's define an actual symbol to make grepping and
understanding easier.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>