Follow-up commit to r238220:
Follow-up commit to r238220:

Pass only FEXEC (instead of FREAD|FEXEC) in fgetvp_exec. _fget has to check for
!FWRITE anyway and may as well know about FREAD.

Make _fget code a bit more readable by converting permission checking from if()
to switch(). Assert that correct permission flags are passed.

In collaboration with:	kib
Approved by:	trasz (mentor)
MFC after:	6 days
X-MFC: with r238220

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase