[RFC] use a shared lock for VOP_GETEXTATTR

VOP_GETEXTATTR is currently called with an exclusive lock, which seems
like overkill for what is essentially a read operation.  I had a look
over the various in-tree filesystems and it didn't look like any of
them will have a problem if a shared-mode lock is used for
vop_getextattr.

Does anyone know otherwise?  Is someone using extended attributes
regularly who can test this?

[sorry if this is a duplicate; I first sent from my non-FreeBSD mail]

Thanks,
matthew