xfs: allow userspace to rebuild metadata structures

xfs: allow userspace to rebuild metadata structures

Source kernel commit: 5c83df2e54b6af870e3e02ccd2a8ecd54e36668c

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>
Signed-off-by: Carlos Maiolino <cem@kernel.org>