LU-12313 llite: Mark lustre_inode_cache as reclaimable

LU-12313 llite: Mark lustre_inode_cache as reclaimable

This is required for proper kernel memory available accounting.
Without it memory allocated to lustre_inode_cache appears as
SUnreclaim where in reality it should apper as SReclaimable.
This affect MemAvailable as well (it is lower than it should be).

Signed-off-by: Jacek Tomaka <jacek.tomaka@poczta.fm>
Change-Id: Iac526a62d0e063b82eea451d1fafa42f2bb4d3b8
Reviewed-on: https://review.whamcloud.com/35790
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>