ceph: fix uninitialized dentry pointer in ceph_real_mount()

ceph: fix uninitialized dentry pointer in ceph_real_mount()

    fs/ceph/super.c: In function ‘ceph_real_mount’:
    fs/ceph/super.c:818: warning: ‘root’ may be used uninitialized in this function

If s_root is already valid, dentry pointer root is never initialized,
and returned by ceph_real_mount(). This will cause a crash later when
the caller dereferences the pointer.

Fixes: 2dcc37d35e7a8988 ("ceph: avoid accessing / when mounting a subpath")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Yan, Zheng <zyan@redhat.com>