xfs: avoid mount-time deadlock in CoW extent recovery

xfs: avoid mount-time deadlock in CoW extent recovery

Source kernel commit: 3ecb3ac7b950ff8f6c6a61e8b7b0d6e3546429a0

If a malicious user corrupts the refcount btree to cause a cycle between
different levels of the tree, the next mount attempt will deadlock in
the CoW recovery routine while grabbing buffer locks.  We can use the
ability to re-grab a buffer that was previous locked to a transaction to
avoid deadlocks, so do that here.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>