xfs: rename struct xfs_eofblocks to xfs_icwalk

The xfs_eofblocks structure is no longer well-named -- nowadays it
provides optional filtering criteria to any walk of the incore inode
cache.  Only one of the cache walk goals has anything to do with
clearing of speculative post-EOF preallocations, so change the name to
be more appropriate.

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