perf: problem of 32bit mmap call graph recording

[Lin Ming]

Hi all, 

On 32 bit machine, we have problem to get the usersapce call graph of
mmap syscall.

You can reproduce it with below command on 32bit machine.

sudo perf record -f -g -e raw_syscalls:sys_enter --filter "id==192" ls

sudo perf report |less

# Events: 26 
#
# Overhead  Command      Shared Object  Symbol
# ........  .......  .................  ......
#
   100.00%       ls  [kernel.kallsyms]  [k] syscall_trace_enter
                 |
                 --- syscall_trace_enter
                     syscall_trace_entry
                    |          
                    |--88.46%-- __mmap  <--- only __mmap, no more userspace call stacks
                    |          
                     --11.54%-- 0xffffe424


Kernel relies on userspace bp register to get call graph information.
But for mmap syscall, the bp register is used to pass in the sixth
parameter(offset).

void *mmap(void *addr, size_t length, int prot, int flags,
                  int fd, off_t offset);

Anyway to fix this issue on 32bit machine?

Thanks,
Lin Ming

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