xfs: allow userspace to rebuild metadata structures

Add a new (superuser-only) flag to the online metadata repair ioctl to
force it to rebuild structures, even if they're not broken.  We will use
this to move metadata structures out of the way during a free space
defragmentation operation.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>