LU-8056 llite: use inode_lock to access i_mutex

LU-8056 llite: use inode_lock to access i_mutex

Linux kernel 4.5 introduced wrappers for i_mutex as the type
of i_mutex will be changed in the future.
This patch prepares us for the upcoming type change. It also
includes a patch from linux kernel removing unused declaration
to make the room for the wrappers mentioned above.

Note that this patch only touches the llite part. osd-ldiskfs still
calls mutex_{lock,unlock} on i_mutex.

Linux-commit:57b8f112cfe6622ddddb8c2641206bb5fa8a112d
Linux-commit:5955102c9984fa081b2d570cfac75c97eecf8f3b

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Change-Id: Ia4c30443cddfaa5232e1dc1519c33c97dba2a271
Reviewed-on: http://review.whamcloud.com/20223
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>