signalfd: Remove SIL_PERF_EVENT fields from signalfd_siginfo

signalfd: Remove SIL_PERF_EVENT fields from signalfd_siginfo

With the addition of ssi_perf_data and ssi_perf_type struct signalfd_siginfo
is dangerously close to running out of space.  All that remains is just
enough space for two additional 64bit fields.  A practice of adding all
possible siginfo_t fields into struct singalfd_siginfo can not be supported
as adding the missing fields ssi_lower, ssi_upper, and ssi_pkey would
require two 64bit fields and one 32bit fields.  In practice the fields
ssi_perf_data and ssi_perf_type can never be used by signalfd as the signal
that generates them always delivers them synchronously to the thread that
triggers them.

Therefore until someone actually needs the fields ssi_perf_data and
ssi_perf_type in signalfd_siginfo remove them.  This leaves a bit more room
for future expansion.

v1: https://lkml.kernel.org/r/20210503203814.25487-12-ebiederm@xmission.com
v2: https://lkml.kernel.org/r/20210505141101.11519-12-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20210517195748.8880-5-ebiederm@xmission.com
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>