xfs: don't fail xfs_extent_busy allocation

xfs: don't fail xfs_extent_busy allocation

BugLink: http://bugs.launchpad.net/bugs/1681875
commit 5e30c23d13919a718b22d4921dc5c0accc59da27 upstream.

We don't just need the structure to track busy extents which can be
avoided with a synchronous transaction, but also to keep track of
pending discard.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>