tlbmod exception panics. This appears to have been incorrect even in the pica
pmap.c, which I pulled in for the pmax. Fixes Xcfbpmax crashes on R40x0
systems with sfb framebuffer.
Fix resident_count tracking so it doesn't go negative. UVM will change a
page mapping without removing the original mapping. That was causing a
problem early on when getting UVM working on the mips, and the explicit
pmap_remove() done by pmap_enter() was decrementing the resident_count.
Do the pmap_remove() first, then check if the current mapping is not valid
and increment the resident_count.
263d2c47a5