MFC r257154:

Merging local and remote bitmaps must be protected by hr_amp lock.

This is believed to fix hastd crashes, which might occur during
synchronization, triggered by the failed assertion:

 Assertion failed: (amp->am_memtab[ext] > 0),
 function activemap_write_complete, file activemap.c, line 351.

Approved by:	re (glebius)