extattr_list_vp: Only take shared vnode lock

[cem]

Summary

List is a 'read'-type operation that does not modify shared state; it's safe
for multiple thread to proceed concurrently. This is reflected in the vnode
operation LISTEXTATTR locking protocol specification, which only requires a
shared lock.

(Similar to previous r248933.)

Reported by: Case van Rij <case.vanrij AT isilon.com>

Test Plan

Suggestions welcome

[mjg]

Lock acquisition can be moved few lines later, after branching on data. Similarly, unlock can be done earlier.

[cem]

In D19082#408030, @mjg wrote:

Lock acquisition can be moved few lines later, after branching on data. Similarly, unlock can be done earlier.

Ok, will do.