xfs: pass the goal of the incore inode walk to xfs_inode_walk()

As part of removing the indirect calls and radix tag implementation
details from the incore inode walk loop, create an enum to represent the
goal of the inode iteration.  More immediately, this separate removes
the need for the "ICI_NOTAG" define which makes little sense.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>