Switch geom_disk over to using a pool mutex.

The GEOM disk d_mtx is only acquired on disk creation and destruction.
It is a good candidate for replacement with a pool mutex. This eliminates
the mutex initialization and teardown and the mutex and name variables
themselves from struct disk.

sys/geom/geom_disk.h:
Take d_mtx and d_mtx_name out of struct disk.

sys/geom/geom_disk.c:
Use mtx_pool_lock() and mtx_pool_unlock() to guard the disk
initialization state instead of a dedicated mutex.

This allows removing the initialization and destruction of
d_mtx.

sys/sys/param.h:
Bump __FreeBSD_version to 1100119 for the change to struct disk.

Suggested by: jhb
Sponsored by: Spectra Logic
Approved by: re (gjb)