Fix leak in libc NFSv4 ACL flags parsing

Free memory allocated by strdup() in parse_flags_verbose().

Submitted by: Andrew Walker <walker.aj325_gmail.com>
Reported by: valgrind
Reviewed by: allanjude, freqlabs, rpokala
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D29871

(cherry picked from commit dd559118ab3f065ae230b324c3fe4bb1c9b7ca56)