staging/lustre/llite: Get rid of ll_lock_dcache/ll_unlock_dcache

These are just doing spin_lock/unlock on inode's i_lock,
so just do the spinlock directly to make the code more clear

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>