dtrace: fix fbt regression for aarch64

fbt computes incorrect instruction position for AArch64 kernel module symbol.

The issue is with the for loop, it does an extra increment of instr pointer
after the required instruction is found. Hence, a wrong instruction is
targeted for patching.

Signed-off-by: Igor Ostapenko <pm@igoro.pro>

Approved by: re (gjb)
Fixes: 980746e5cb26 ("fbt: simplify arm64 function-prologue parsing")
Reviewed by: markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/855
MFC after: 1 week

(cherry picked from commit b4db386f9fa7be9668aa09d14b0bbd048a7a7e89)
(cherry picked from commit 2ba605f32ac3c673ef0f0bee810a2696f1887488)