extattr_list_vp: Only take shared vnode lock

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>
Reviewed by: kib, mjg
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D19082