UPSTREAM: f2fs: fix to use mnt_{want,drop}_write_file replace file_{start,end}_wrtie

mnt_{want,drop}_write_file is more suitable than
file_{start,end}_wrtie and also is consistent with
other ioctl operations.

Change-Id: I9491200c75a56778be4afbd2a9744b462d2c7ba2
Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 1efb7c8f)