gfs2: be careful with inode refresh

1) gfs2_dinode_in() should *not* touch ->i_rdev on live inodes; even
"zero and immediately reread the same value from dinode" is broken -
have it overlap with ->release() of char device and you can get all
kinds of bogus behaviour.

2) mismatch on inode type on live inodes should be treated as fs
corruption rather than blindly setting ->i_mode.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>