xtensa: save current register frame in fast_syscall_spill_registers_fixup

We need it saved because it contains a3 where we track which register
windows we still need to spill, and fixup handler may call C exception
handlers. Also fix comments.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

diff --cc arch/xtensa/kernel/entry.S
Simple merge