LU-7973 llite: lock i_lock/dcache_lock before __d_drop()

LU-7973 llite: lock i_lock/dcache_lock before __d_drop()

There has been several Lustre Client crashes reported by sites
running with Lustre versions 2.1/2.5, all showing the same
dentry->d_hash->next corrupted pointer cause.

This patch fixes a regression that has been introduced since a
long time by commit :
d10c9e05ab593f361fdfd27842766a1924e63e58
(LU-506 kernel: FC15 - support dcache scalability changes.)

where i_lock/dcache_lock protection usage has been removed and
that is likely to cause racy condition during dentry [un]hashing
and to be the root cause of these crashes.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ie1239058222f15f7e094d8fae4d37e4a36449707
Reviewed-on: http://review.whamcloud.com/19287
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>