Add pmap locking to many of the functions.

Add pmap locking to many of the functions.

Implement the protection check required by the pmap_extract_and_hold()
specification.

Remove the acquisition and release of Giant from pmap_extract_and_hold() and
pmap_protect().

Many thanks to Ken Smith for resolving a sparc64-specific initialization
problem in my original patch.

Tested by: kensmith@