libc: do not zero errno on success in vfs plugin

The manpage to errno tells the following story:

The <errno.h> header file defines the integer variable errno, which is
set by system calls and some library functions in the event of an error
to indicate what went wrong. Its value is significant only when the
return value of the call indicated an error (i.e., -1 from most system
calls; -1 or NULL from most library functions); a function that
succeeds is allowed to change errno.

Valid error numbers are all nonzero; errno is never set to zero by any
system call or library function.