instead of calling pmap_invalidate_page() for each PG_G mapping, call
pmap_invalidate_range() for each range of PG_G mappings. In addition,
eliminate a redundant call to pmap_invalidate_page(). Both
pmap_remove_pte() and pmap_remove_page() called pmap_invalidate_page()
when the mapping had the PG_G attribute. Now, only pmap_remove_page()
calls pmap_invalidate_page(). Altogether, these changes eliminate 53%
of the TLB shootdowns for a "buildworld" on a ZFS file system. On
FFS, the reduction is 3%.
MFC after: 6 weeks
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