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Add a "count_until_fail" option to gnop, which says to start failing

Description

Add a "count_until_fail" option to gnop, which says to start failing
I/O requests after the given number have been allowed though.

Approved by: imp (mentor)
Reviewed by: rpokala kib 0mp mckusick
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21593