xfs: Decouple XFS_IBULK flags from XFS_IWALK flags

A future commit will add a new XFS_IBULK flag which will not have a
corresponding XFS_IWALK flag. In preparation for the change, this commit
separates XFS_IBULK_* flags from XFS_IWALK_* flags.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>