About function __create_file in debugfs

[Jianpeng Ma]

Hi:
	At present,i used blktrace to trace block io.But i always met error, the message like:
>BLKTRACESETUP(2) /dev/sdc failed: 2/No such file or directory
>Thread 0 failed open /sys/kernel/debug/block/(null)/trace0: 2/No such file or directory
>Thread 2 failed open /sys/kernel/debug/block/(null)/trace2: 2/No such file or directory
>Thread 3 failed open /sys/kernel/debug/block/(null)/trace3: 2/No such file or directory

>Thread 1 failed open /sys/kernel/debug/block/(null)/trace1: 2/No such file or directory
>FAILED to start thread on CPU 0: 1/Operation not permitted
>FAILED to start thread on CPU 1: 1/Operation not permitted
>FAILED to start thread on CPU 2: 1/Operation not permitted
>FAILED to start thread on CPU 3: 1/Operation not permitted

But those isn't important. I add some message in kernel and found the reason is inode already existed.
But the function __create_file dosen't return correctly errno.So blktrace tool can't print correctly message.
I think func __create_file should return correctly message(ERR_PTR(error)) not NULL.
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