During F_SETFL, don't change file flags on error

Previously, even if the FIONBIO or FIOASYNC ioctl failed, the file's
f_flags variable would still be changed. Now, kern_fcntl will restore
the original flags if the ioctl fails.

PR: 265736
Reported by: Yuval Pavel Zholkover <paulzhol@gmail.com>
MFC after: 2 weeks
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D40955