Three improvements to vnode_pager_generic_getpages():

Eliminate an exclusive object lock acquisition and release on the expected
execution path.

Do page zeroing before the object lock is acquired rather than during the
time that the object lock is held.

Use vm_pager_free_nonreq() to eliminate duplicated code.

Reviewed by: kib
MFC after: 6 weeks
Sponsored by: EMC / Isilon Storage Division