libdm: do not fail if GETVAL semctl fails for udev sync inc and dec

libdm: do not fail if GETVAL semctl fails for udev sync inc and dec

While performing udev sync semaphore's inc/dec operation, we use the
result from GETVAL semctl just to print a debug message with current
value of that sempahore, nothing else.

If the GETVAL fails for whetever reason while the actual inc/dec
completes successfully, just log a warning message about the GETVAL
(and print the debug messages without the actual semaphore value)
and return success for the inc/dec operation as a whole.