linux: Make PTRACE_GET_SYSCALL_INFO handle EJUSTRETURN

This fixes panic when trying to run strace(8) from Focal.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32355

(cherry picked from commit 2558bb8e9166395dce26afcfac91fc692621de7b)