libmount: hide details about failed search in fstab/mtab

The current code returns -errno when not found "mount /foo" in fstab
and mtab does not exist (or /etc/mtab points to non-mounted /proc).

This is problem because the return value is too low-level and maybe
misinterpreted by top level code. It's better to always return
MNT_ERR_NOFSTAB when not found in fstab/mtab.

Reported-by: Dylan Cali <calid1984@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>