posix: eventfd: Fix unsetting internal flags in ioctl

Commit e6eb0a70 ("posix: eventfd: revise locking, signaling, and
allocation") introduced a regression where the internal flags of an
event file descriptor would be erased when calling the F_SETFL ioctl
operation.

This includes the flag EFD_IN_USE_INTERNAL which determines whether
this file descriptor has been opened, thus effectively closing the
eventfd whenever one tries to change a flag.

Signed-off-by: Celina Sophie Kalus <hello@celinakalus.de>
(cherry picked from commit 5bd86ead)