rasdaemon: fix return value type issue of read/write function from unistd.h

rasdaemon: fix return value type issue of read/write function from unistd.h

The return value type of read/write function from unistd.h is ssize_t.
It's signed normally, and return -1 on error. Fix incorrect use in the
function read_ras_event_all_cpus().

BTW, make setting buffer_percent as a separate function.

Fixes: 94750bcf9309 ("rasdaemon: Fix poll() on per_cpu trace_pipe_raw blocks indefinitely")
Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Shiju Jose <shiju.jose@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>