[XFS] Add op_flags field and helpers to xfs_da_args

The end of the xfs_da_args structure has 4 unsigned char fields for
true/false information on directory and attr operations using the
xfs_da_args structure.

The following converts these 4 into a op_flags field that uses the first 4
bits for these fields and allows expansion for future operation
information (eg. case-insensitive lookup request).

SGI-PV: 981520
SGI-Modid: xfs-linux-melb:xfs-kern:31206a

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>