Don't free the cursor boundary tag during vmem_destroy().

The cursor boundary tag is statically allocated in the vmem instead of
from the vmem_bt_zone. Explicitly remove it from the vmem's segment
list in vmem_destroy before freeing all the segments from the vmem.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D21953