ksmbd: fix __write_overflow warning in ndr_read_string

Dan reported __write_overflow warning in ndr_read_string.

  CC [M]  fs/ksmbd/ndr.o
In file included from ./include/linux/string.h:253,
                 from ./include/linux/bitmap.h:11,
                 from ./include/linux/cpumask.h:12,
                 from ./arch/x86/include/asm/cpumask.h:5,
                 from ./arch/x86/include/asm/msr.h:11,
                 from ./arch/x86/include/asm/processor.h:22,
                 from ./arch/x86/include/asm/cpufeature.h:5,
                 from ./arch/x86/include/asm/thread_info.h:53,
                 from ./include/linux/thread_info.h:60,
                 from ./arch/x86/include/asm/preempt.h:7,
                 from ./include/linux/preempt.h:78,
                 from ./include/linux/spinlock.h:55,
                 from ./include/linux/wait.h:9,
                 from ./include/linux/wait_bit.h:8,
                 from ./include/linux/fs.h:6,
                 from fs/ksmbd/ndr.c:7:
In function memcpy,
    inlined from ndr_read_string at fs/ksmbd/ndr.c:86:2,
    inlined from ndr_decode_dos_attr at fs/ksmbd/ndr.c:167:2:
./include/linux/fortify-string.h:219:4: error: call to __write_overflow
declared with attribute error: detected write beyond size of object
    __write_overflow();
    ^~~~~~~~~~~~~~~~~~

This seems to be a false alarm because hex_attr size is always smaller
than n->length. This patch fix this warning by allocation hex_attr with
n->length.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>