Pull vm_object_scan_all_shadowed out of vm_object_backing_scan

These two functions were largely unrelated, they just used the same same
loop logic to walk through a backing object's memq. Pull out the
all_shadowed test as its own function and eliminate
OBSC_TEST_ALL_SHADOWED. Rename vm_object_backing_scan to
vm_object_collapse_scan.

No functional change.

Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4335