ext2: Set superblock revision when enabling xattr feature

When setting the first xattr, we automatically enable
EXT2_FEATURE_COMPAT_EXT_ATTR. However we forget to call
ext2_update_dynamic_rev() so in theory if the filesystem was created as
ancient one without features support, this could be missed. The
consequences are minor anyway - since the feature is compat one, only
old e2fsck which does not understand xattrs could do something bad.

Reported-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>