ARM: 7525/1: ptrace: use updated syscall number for syscall auditing

When tracing system calls, a debugger may change the syscall number
in response to a SIGTRAP on syscall entry.

This patch ensures that the new syscall number is passed to the audit
code.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>