linux: Make PTRACE_GETREGSET return proper buffer size

This fixes Chrome warning:

[1022/152319.328632:ERROR:ptracer.cc(476)] Unexpected registers size 0 != 216, 68

Reviewed By: emaste
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32616

(cherry picked from commit f939dccfd770df2f9070cd30aa52105b7afe1bde)