Remove page locking from pmap_mincore().

Remove page locking from pmap_mincore().

After r352110 the page lock no longer protects a page's identity, so
there is no purpose in locking the page in pmap_mincore().  Instead,
if vm.mincore_mapped is set to the non-default value of 0, re-lookup
the page after acquiring its object lock, which holds the page's
identity stable.

The change removes the last callers of vm_page_pa_tryrelock(), so
remove it.

Reviewed by:	kib
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D21823