Currently, an NFSv3 ACCESS check for READ permission on an
eXecute-only file will succeed where it should fail.

This is because nfsd_permission allows READ access to eXecute only
files so that mode 711 executables can be loaded and run, and
nfsd_access simply uses nfsd_permission.

This patch changes nfsd_permission to only map eXecute permission to
read permission of MAY_OWNER_OVERRIDE was set.  This is only set
when trying to read from a file, so ACCESS will no longer be tricked.

This change will only affect callers of nfsd_permission that specify
MAY_READ and not MAY_OWNER_OVERRIDE, and nfsd_access is the only
routine that calls nfsd_permission (via fh_verify) that way.