sys_remap_file_pages: fix ->vm_file accounting

patch 8a459e44 in mainline.

Fix ->vm_file accounting, mmap_region() may do do_munmap().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>