the argument passed to file_dup2 doesn't always come from task file list
so it doesn't make sense to hold the file list lock and then it is better
to do the protection in the new function files_dupfd2
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ibf02cea9b0b275e7472f9c04fd66b9242285b957