fs: cifs: use kmemdup instead of kmalloc + memcpy

This replaces calls to kmalloc followed by memcpy with a single call to
kmemdup. This was found via make coccicheck.

Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Steve French <smfrench@gmail.com>