xfs: clean up xfs_inactive() error handling, kill VN_INACTIVE_[NO]CACHE

The xfs_inactive() return value is meaningless. Turn xfs_inactive()
into a void function and clean up the error handling appropriately.
Kill the VN_INACTIVE_[NO]CACHE directives as they are not relevant
to Linux.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>