vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Conflicts:
	fs/ext4/ioctl.c
	fs/xfs/xfs_ioctl.c

Change-Id: Idcde3c3eba747966dcfc692f1a2f473cbfdf050a