perf trace: Check and discard not only 'nr' but also '__syscall_nr'

perf trace: Check and discard not only 'nr' but also '__syscall_nr'

Format fields of a syscall have the first variable '__syscall_nr' or
'nr' that mean the syscall number.  But it isn't relevant here so drop
it.

'nr' among fields of syscall was renamed '__syscall_nr'.  So add
exception handling to drop '__syscall_nr' and modify the comment for
this excpetion handling.

Reported-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Taeung Song <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>