perf_event: Make perf fd non seekable

perf_event: Make perf fd non seekable

Perf_event does not need seeking, so prevent it in order to
get rid of default_llseek, which uses the BKL.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
[drop the nonseekable_open, not needed for anon inodes]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>