fcntl(F_SETFL): only allow one thread to perform F_SETFL

Use f_vflags file locking for this.
Allowing more than one thread handling F_SETFL might cause de-sync
between real driver state and flags.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D52487