perf trace beauty: Always show mmap prot even though PROT_NONE

perf trace beauty: Always show mmap prot even though PROT_NONE

PROT_NONE is also useful information, so do not omit the mmap prot even
though it is 0. syscall_arg__scnprintf_mmap_prot() could print PROT_NONE
for prot 0.

Before: PROT_NONE is not shown.
$ sudo perf trace -e syscalls:sys_enter_mmap --filter prot==0  -- ls
     0.000 ls/2979231 syscalls:sys_enter_mmap(len: 4220888, flags: PRIVATE|ANONYMOUS)

After: PROT_NONE is displayed.
$ sudo perf trace -e syscalls:sys_enter_mmap --filter prot==0  -- ls
     0.000 ls/2975708 syscalls:sys_enter_mmap(len: 4220888, prot: NONE, flags: PRIVATE|ANONYMOUS)

Signed-off-by: Changbin Du <changbin.du@huawei.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240522033542.1359421-3-changbin.du@huawei.com