LU-8891 llite: rcu-walk check should not depend on statahead

LU-8891 llite: rcu-walk check should not depend on statahead

Moves the check for the LOOKUP_RCU flag, so that it does not depend
on the statahead setting.  The caller is now informed if rcu-walk
was requested but the filesystem does not support it, regardless
of whether statahead is enabled or disabled.

Signed-off-by: Steve Guminski <stephenx.guminski@intel.com>
Change-Id: I0dcfd594c1b44cba1741c60a24ed623ef635b79c
Reviewed-on: https://review.whamcloud.com/24195
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>