erofs: accept source file descriptor via fsconfig

Allow userspace to pass an already-opened file descriptor as the mount
source instead of a path string.  This is useful for tools that already
hold an fd to the image, such as composefs reusing an existing erofs
backing file.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>