xfs: make the icwalk processing functions clean up the grab state

Soon we're going to be adding two new callers to the incore inode walk
code: reclaim of incore inodes, and (later) inactivation of inodes.
Both states operate on inodes that no longer have any VFS state, so we
need to move the xfs_irele calls into the processing functions.

In other words, icwalk processing functions are responsible for cleaning
up whatever state changes are made by the corresponding icwalk igrab
function that picked the inode for processing.

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