ceph: don't allow type or device number to change on non-I_NEW inodes

Al pointed out that a malicious or broken MDS could change the type or
device number of a given inode number. It may also be possible for the
MDS to reuse an old inode number.

Ensure that we never allow fill_inode to change the type part of the
i_mode or the i_rdev unless I_NEW is set. Throw warnings if the MDS ever
changes these on us mid-stream, and return an error.

Don't set i_rdev directly, and rely on init_special_inode to do it.
Also, fix up error handling in the callers of ceph_get_inode.

In handle_cap_grant, check for and warn if the inode type changes, and
only overwrite the mode if it didn't.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>