perf buildid-cache: Zero out buffer of filenames when adding/removing buildid

The readlink() function doesn't append a null byte to buf. So we should
zero out buf with zalloc(). Or we'll see sometimes error like this:

[root@intel-s3e36-01]~# /usr/bin/perf buildid-cache -a /lib/modules/2.6.32-130.el6.x86_64/kernel/crypto/twofish_common.ko -v
Adding f64ba8efd5f53c7ad332fc17db1d21de309038e1 /lib/modules/2.6.32-130.el6.x86_64/kernel/crypto/twofish_common.ko: Ok
[root@intel-s3e36-01]~# /usr/bin/perf buildid-cache -r /lib/modules/2.6.32-130.el6.x86_64/kernel/crypto/twofish_common.ko -v
Removing f64ba8efd5f53c7ad332fc17db1d21de309038e1 /lib/modules/2.6.32-130.el6.x86_64/kernel/crypto/twofish_common.ko: FAIL
/lib/modules/2.6.32-130.el6.x86_64/kernel/crypto/twofish_common.ko wasn't in the cache

The change in build_id_cache__add_s() is a defense.

Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20110718031314.GA5802@hpt.nay.redhat.com
Signed-off-by: Han Pingtian <phan@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>