Fix M_GTASKQUEUE definition

Previously had the same short and long description as taskqueues.
This could cause problems with memguard(9) and vmstat -m which use
the short description as a unique identifier.

Reviewed by: sbruno
Approved by: sbruno (mentor)
MFC after: 3 days
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12438