__get_str(msg) does not need (char *) operator overloading to access
mgs's elements anymore. This patch substitutes
((char *)__get_str(msg))[0] usage to __get_str(msg)[0].

It is just a code cleanup, no changes on tracepoint ABI.

Link: http://lkml.kernel.org/r/6f2db5be7705da2cb483923320c91283d7c712a7.1467407618.git.bristot@redhat.com

Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: Ingo Molnar <mingo@redhat.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>