ext4: fix bogus warning in ext4_update_dx_flag()

The idea of the warning in ext4_update_dx_flag() is that we should warn
when we are clearing EXT4_INODE_INDEX on a filesystem with metadata
checksums enabled since after clearing the flag, checksums for internal
htree nodes will become invalid. So there's no need to warn (or actually
do anything) when EXT4_INODE_INDEX is not set.

Link: https://lore.kernel.org/r/20201118153032.17281-1-jack@suse.cz
Fixes: 48a34311

 ("ext4: fix checksum errors with indexed dirs")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org