Don't allow FBT to recurse into FBT functions when DTrace is built as part of the kernel

[rwatson]

Summary

Normally, DTrace avoids recursion into itself by recognising functions in its own kernel module. On ARMv[67], when building DTrace into the kernel itself, the normal further rule of not entering functions prefixed with dtrace_ is insufficient for FBT, which also contains functions prefixed with fbt_. This change blocks that recursion by disallowing FBT instrumentation of fbt_ functions.

Test Plan

Deployed in Cambridge L41 2014-2015.

[rwatson]

Updated patch generated against Subversion, and contains more context. No functional change.

[andrew]

This should be replaced with fbt_excluded. Ruslan added it for arm64, but it doesn't seem to be used elsewhere. This will mean we can remove the dtrace_ and __ cased above.