perf/core: Remove optional 'size' arguments from strscpy() calls

The 'size' parameter is optional and strscpy() automatically determines
the length of the destination buffer using sizeof() if the argument is
omitted. This makes the explicit sizeof() calls unnecessary.

Furthermore, KSYM_NAME_LEN is equal to sizeof(name) and can also be
removed. Remove them to shorten and simplify the code.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250310192336.442994-1-thorsten.blum@linux.dev