User can remove files from cache using --remove/--purge options but both
needs list of files as an argument. It's not convenient when you want to
flush out entire cache. Add an option to purge all files from cache.

Ex,
  # perf buildid-cache -l
    8a86ef73e44067bca52cc3f6cd3e5446c783391c /tmp/a.out
    ebe71fdcf4b366518cc154d570a33cd461a51c36 /tmp/a.out.1
  # perf buildid-cache -P -v
    Removing /tmp/a.out (8a86ef73e44067bca52cc3f6cd3e5446c783391c): Ok
    Removing /tmp/a.out.1 (ebe71fdcf4b366518cc154d570a33cd461a51c36): Ok
    Purged all: Ok

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180417041346.5617-4-ravi.bangoria@linux.vnet.ibm.com


[ Initialize 'err' in build_id_cache__purge_all(), to fix build on debian:7, as it can be used uninitialized ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>