Initialize the eflags field of vm_map headers.

Initializing the eflags field of the map->header entry to a value with a
unique new bit set makes a few comparisons to &map->header unnecessary.

Submitted by: Doug Moore <dougm@rice.edu>
Reviewed by: alc, kib
Tested by: pho
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14005