kernel: mmu: move when page fault is counted

The beginning of code in do_page_fault() is to pin the page
in memory if it is already present in physical memory.
It is there so that if a page is not present, it can proceed
to perform page-in and then pin it. So the counting of
page faults needs to be moved after the pinning code so
it actually counts page faults, and not counting pinning
operations when the page is already present.

Also clarify the comment on the goto statement as it is not
correct.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>