geom: Add counts for enomem and pausing

Add counts for the number of requests that complete with the ENOMEM as
kern.geom.nomem_count and the number of times we pause the g_down thread
to let the system recover as kern.geom.pause_count.

Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45309