iomap support for fuse is also going to want the ability to attach
backing files to a fuse filesystem. Move the fuse_backing code into a
separate file so that both can use it.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
