staging: lustre: use memdup_user to allocate memory and copy from user

Replace a call to kmalloc and a call to copy_from_user with a
call to memdup_user to simplify the code.
Issue found with coccicheck.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>