tracing/fastboot: fix len of func buffer

Impact: fix possible stack overrun

This is a port of a patch included in the mainline (KSYM_SYMBOL_LEN fixes).
The current func len is not large enough to contain the max symbol len, the
right size must be KSYM_SYMBOL_LEN.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>