ntfs3: avoid memcpy size warning

There are more entries after the structure, use unsafe_memcpy() to avoid
this warning.

syzbot reported:
memcpy: detected field-spanning write (size 3656) of single field "hdr1" at fs/ntfs3/index.c:1927 (size 16)
Call Trace:
indx_insert_entry+0x1a0/0x460 fs/ntfs3/index.c:1996
ni_add_name+0x4dd/0x820 fs/ntfs3/frecord.c:2995
ni_rename+0x98/0x170 fs/ntfs3/frecord.c:3026
ntfs_rename+0xab9/0xf00 fs/ntfs3/namei.c:332

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=3a1878433bc1cb97b42a
Signed-off-by: Lizhi Xu <[e-mail]>
Signed-off-by: Konstantin Komarov <[e-mail]>

fs/ntfs3/index.c

···
 		 * Undo critical operations.
 		 */
 		indx_mark_free(indx, ni, new_vbn >> indx->idx2vbn_bits);
-		memcpy(hdr1, hdr1_saved, used1);
+		unsafe_memcpy(hdr1, hdr1_saved, used1,
+			      "There are entries after the structure");
 		indx_write(indx, ni, n1, 0);
 	}