tracing: Fix return value of trace_pid_write()

tracing: Fix return value of trace_pid_write()

Setting set_event_pid with trailing whitespace lead to endless write
system calls like below.

    $ strace echo "123 " > /sys/kernel/debug/tracing/set_event_pid
    execve("/usr/bin/echo", ["echo", "123 "], ...) = 0
    ...
    write(1, "123 \n", 5)                   = 4
    write(1, "\n", 1)                       = 0
    write(1, "\n", 1)                       = 0
    write(1, "\n", 1)                       = 0
    write(1, "\n", 1)                       = 0
    write(1, "\n", 1)                       = 0
    ....

This is because, the result of trace_get_user's are not returned when it
read at least one pid. To fix it, update read variable even if
parser->idx == 0.

The result of applied patch is below.

    $ strace echo "123 " > /sys/kernel/debug/tracing/set_event_pid
    execve("/usr/bin/echo", ["echo", "123 "], ...) = 0
    ...
    write(1, "123 \n", 5)                   = 5
    close(1)                                = 0

Link: https://lkml.kernel.org/r/[email protected]
Cc: Ingo Molnar <[email protected]>
Cc: Baik Song An <[email protected]>
Cc: Hong Yeon Kim <[email protected]>
Cc: Taeung Song <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: 4909010788640 ("tracing: Add set_event_pid directory for future use")
Signed-off-by: Wonhyuk Yang <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>