Fix incorrect mount table entry parsing in __getmntent_r

When mount entry contains only four fields and have more then one space or
tab at the and, mp.mnt_freq and mp.mnt_passno will be set to some specific
values as side effect from parsing of previus mount entry. It is because
sscanf(""," %d %d ", &a, &b) returns -1, but this case is unprocessed.
Values of mp.mnt_freq and  mp.mnt_passno stays unchanged. This patch is
attempt to fix described issue by removing trailing tabs and spaces.