Each WRITE request must carry a valid file descriptor.  When a page is written
back from a memory mapping, the file through which the page was dirtied is not
available, so a new mechananism is needed to find a suitable file in
->writepage(s).

A list of fuse_files is added to fuse_inode.  The file is removed from the
list in fuse_release().

This patch is in preparation for writable mmap support.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>