fuse: do not allow mapping a non-regular backing file

We do not support passthrough operations other than read/write on
regular file, so allowing non-regular backing files makes no sense.

Fixes: efad7153 ("fuse: allow O_PATH fd for FUSE_DEV_IOC_BACKING_OPEN")
Cc: stable@vger.kernel.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>