Fix leak in libc NFSv4 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
MFC after: 3 days
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D29871