[XFS] Clean up some existing compat ioctl calls

Create a new xfs_ioctl.h file which has prototypes for
ioctl helpers that may be called in compat mode.

Change several compat ioctl cases which are IOW to simply copy
in the userspace argument, then call the common ioctl helper.

This also fixes xfs_compat_ioc_fsgeometry_v1(), which had
it backwards before; it copied in an (empty) arg, then copied
out the native result, which probably corrupted userspace.  It
should be translating on the copyout.

Also, a bit of formatting cleanup for consistency, and conversion
of all error returns to use XFS_ERROR().

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>