ext4: Check that external xattr value block is zero

Currently we don't support xattrs with values stored out of line. Check
for that in ext4_xattr_check_names() to make sure we never work with
such xattrs since not all the code counts with that resulting is possible
weird corruption issues.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>