[XFS] resync headers with libxfs

 - xfs_sb.h add the XFS_SB_VERSION2_PARENTBIT features2 that has been
   around in userspace for some time
 - xfs_inode.h: move a few things out of __KERNEL__ that are needed by
   userspace
 - xfs_mount.h: only include xfs_sync.h under __KERNEL__
 - xfs_inode.c: minor whitespace fixup.  I accidentaly changes this when
   importing this file for use by userspace.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>