USB: gadget: f_fs: fix memdup_user.cocci warnings

drivers/usb/gadget/function/f_fs.c:3829:8-15: WARNING opportunity for memdup_user

 Use memdup_user rather than duplicating its implementation
 This is a little bit restricted to reduce false positives

Generated by: scripts/coccinelle/api/memdup_user.cocci

Fixes: 8704fd73 ("USB: gadget: f_fs: remove likely/unlikely")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20210308070951.GA83949@8a16bdd473dcSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -3827,14 +3827,9 @@ static char *ffs_prepare_buffer(const char __user *buf, size_t len)
 	if (!len)
 		return NULL;
 
-	data = kmalloc(len, GFP_KERNEL);
-	if (!data)
-		return ERR_PTR(-ENOMEM);
-
-	if (copy_from_user(data, buf, len)) {
-		kfree(data);
-		return ERR_PTR(-EFAULT);
-	}
+	data = memdup_user(buf, len);
+	if (IS_ERR(data))
+		return ERR_PTR(PTR_ERR(data));
 
 	pr_vdebug("Buffer from user space:\n");
 	ffs_dump_mem("", data, len);