coda: use vmemdup_user to replace the open code

vmemdup_user is better than duplicating its implementation, So just
replace the open code.

  fs/coda/psdev.c:125:10-18:WARNING:opportunity for vmemdup_user

The issue is detected with the help of Coccinelle.

Link: https://lkml.kernel.org/r/20210908140308.18491-9-jaharkes@cs.cmu.edu


Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yangyang <jing.yangyang@zte.com.cn>
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Xin Tan <tanxin.ctf@gmail.com>
Cc: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>