fs/lock: documentation cleanup. Replace inode->i_lock with flc_lock.

Update lock usage of lock_manager_operations' functions to reflect
the changes in commit 6109c850

 ("locks: add a dedicated spinlock
to protect i_flctx lists").

Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>