[XFS] attrmulti cleanup

xfs_attrmulti_by_handle currently request the size based on
sizeof(attr_multiop_t) but should be using sizeof(xfs_attr_multiop_t)
because that is what it is dealing with. Despite beeing wrong this
actually harmless in practice because both structures are the same size on
all platforms.

But this sizeof was the only user of struct attr_multiop so we can just
kill it. Also move the ATTR_OP_* defines xfs_attr.h into the struct
xfs_attr_multiop defintion in xfs_fs.h because they are only used with
that structure, and are part of the user ABI for the
XFS_IOC_ATTRMULTI_BY_HANDLE ioctl.

SGI-PV: 983508

SGI-Modid: xfs-linux-melb:xfs-kern:31352a

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