include/wmutex: Don't write to stderr when wmutex_unlock() fails

include/wmutex: Don't write to stderr when wmutex_unlock() fails

When a wmutex could not be unlocked, the rm call in wmutex_unlock()
writes an error message directly to stderr. However, the function
should only return an error without writing to stderr.

This commit modifies wmutex_unlock() so that it does not write to
stderr.