perf record: Simplify perf_record__write

1. Since all callers either test if it is less than zero or assign its
   result to an int variable, convert it from ssize_t to int;

2. There is just one use for the 'session' variable, so use rec->session
   directly instead;

3. No need to store the result of perf_data_file__write, since that
   result is either 'size' or -1, the later making the error result to
   be stored in 'errno' and accessed thru printf's %m in the pr_err
   call.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-xwsk964dp681fica3xlqhjin@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>