libsysdecode: correctly decode mmap flags

r352913 added decoding of mmap PROT_MAX()'d flags but didn’t account for the
case where different values were specified for PROT_MAX and regular flags.
Fix it.

Submitted by: sigsys_gmail.com
Reported by: sigsys_gmail.com
MFC after: 7 days
Differential Revision: https://reviews.freebsd.org/D27312