Avoid using the 64-bit vm_pindex_t in a few places where 64-bit

Avoid using the 64-bit vm_pindex_t in a few places where 64-bit
types are not required, as the overhead is unnecessary:

 o In the i386 pmap_protect(), `sindex' and `eindex' represent page
   indices within the 32-bit virtual address space.
 o In swp_pager_meta_build() and swp_pager_meta_ctl(), use a temporary
   variable to store the low few bits of a vm_pindex_t that gets used
   as an array index.
 o vm_uiomove() uses `osize' and `idx' for page offsets within a
   map entry.
 o In vm_object_split(), `idx' is a page offset within a map entry.