riscv: trace: fix snapshot deadlock with sbi ecall

In the Linux kernel, the following vulnerability has been resolved:

riscv: trace: fix snapshot deadlock with sbi ecall

If sbi_ecall.c's functions are traceable,

echo "__sbi_ecall:snapshot" > /sys/kernel/tracing/set_ftrace_filter

may get the kernel into a deadlock.

(Functions in sbi_ecall.c are excluded from tracing if
CONFIG_RISCV_ALTERNATIVE_EARLY is set.)

__sbi_ecall triggers a snapshot of the ringbuffer. The snapshot code
raises an IPI interrupt, which results in another call to __sbi_ecall
and another snapshot...

All it takes to get into this endless loop is one initial __sbi_ecall.
On RISC-V systems without SSTC extension, the clock events in
timer-riscv.c issue periodic sbi ecalls, making the problem easy to
trigger.

Always exclude the sbi_ecall.c functions from tracing to fix the
potential deadlock.

sbi ecalls can easiliy be logged via trace events, excluding ecall
functions from function tracing is not a big limitation.