[XFS] Remove CFORK macros and use code directly in IFORK and DFORK macros.

Currently XFS_IFORK_* and XFS_DFORK* are implemented by means of
XFS_CFORK* macros. But given that XFS_IFORK_* operates on an xfs_inode
that embedds and xfs_icdinode_core and XFS_DFORK_* operates on an
xfs_dinode that embedds a xfs_dinode_core one will have to do endian
swapping while the other doesn't. Instead of having the current mess with
the CFORK macros that have byteswapping and non-byteswapping version
(which are inconsistantly named while we're at it) just define each family
of the macros to stand by itself and simplify the whole matter.

A few direct references to the CFORK variants were cleaned up to use IFORK
or DFORK to make this possible.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30163a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>