uncleanly due to a lost connection, it would hang in closef() waiting
for the usecount to go back to 1.
An audit of FILE_USE() vs FILE_UNUSE() usage led me to discover some
incorrect error-path code..
In sys_fcntl(), avoid leaking a file descriptor usecount in an error
case of F_SETFL; don't return, instead go to "out" to clean up. I
suspect that the F_SETFL would fail because vop_fcntl is not
implemented in deadfs.
c3ddfa524d