Fixed issues with uvm_page_physunload_delete_end test case.
1. "avail_start" and "start" were different, resulting in unreachable code in
uvm_page_physunload(), where the condition check "avail_start" < "end" fails.
The test has been fixed by setting "avail_start" and "start" to the same value.
2. If "start" is the address with end address being "start + 2", we can
unplug twice, the first paddr_t would be "start" and the second one would be
"start + 1". Modified the ATF_CHECK_EQ() to reflect these changes.
Reviewed by <cherry>
UnifiedSplitRaw
