perf_event: Fix __dsos__write_buildid_table()

The remain buff size is 'len - pos->long_name_len - 1', not
'len - pos->long_name_len + 1'

This bug was introduced by commit 7691b1ec ("perf tools: Misc small
fixes").

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4B1C7F73.80707@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>