xfs_fsr: fix SWAPEXT failures under selinux

xfs_fsr: fix SWAPEXT failures under selinux

If we run xfs_fsr on a system which creates selinux extended
attributes, the temp file created by xfs_fsr may have a
large-ish local extended attribute as soon as it is created.

If the target file has NON-local extended attributes, it may
have a fork offset larger than the temp file, because i.e.
FMT_EXTENTS attributes take up less space.  We currently
have no mechanism to grow the temp file's fork offset.
So in this case, the SWAPEXT ioctl will fail.

(With systems using selinux and lots of xattrs, this becomes
fairly common in the real world.)

After testing the target file for a non-local extent, and
checking to see if the temp forkoff needs to be grown on the
first pass, we can add a large attr to knock all attributes on
the temp file out of local format, and grow the fork offset for
this particular case.

This passes xfstest 227, and also resolves issues seen on
a metadata image provided by Gabriel.

Reported-by: Gabriel VLASIU <gabriel@vlasiu.net>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>