Remove inclusion of <sys/diskmbr.h>. We have no business knowing
anything related to MBR in this file.
The page hold mechanism is fast but it has couple of fallouts:
- It does not let pages respect the LRU policy
- It bloats the active/inactive queues of few pages

Try to avoid it as much as possible with the long-term target to
completely remove it.
Use the soft-busy mechanism to protect page content accesses during
short-term operations (like uiomove_fromphys()).

After this change only vm_fault_quick_hold_pages() is still using the
hold mechanism for page content access.
There is an additional complexity there as the quick path cannot
immediately access the page object to busy the page and the slow path
cannot however busy more than one page a time (to avoid deadlocks).

Fixing such primitive can bring to complete removal of the page hold
mechanism.

Sponsored by:	EMC / Isilon storage division
Discussed with:	alc
Reviewed by:	jeff
Tested by:	pho
Remove unused member.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	alc
Tested by:	pho