capabilities: fix buffer overread on very short xattr

If userspace attempted to set a "security.capability" xattr shorter than
4 bytes (e.g. 'setfattr -n security.capability -v x file'), then
cap_convert_nscap() read past the end of the buffer containing the xattr
value because it accessed the ->magic_etc field without verifying that
the xattr value is long enough to contain that field.

Fix it by validating the xattr value size first.

This bug was found using syzkaller with KASAN.  The KASAN report was as
follows (cleaned up slightly):

    BUG: KASAN: slab-out-of-bounds in cap_convert_nscap+0x514/0x630 security/commoncap.c:498
    Read of size 4 at addr ffff88002d8741c0 by task syz-executor1/2852

    CPU: 0 PID: 2852 Comm: syz-executor1 Not tainted 4.15.0-rc6-00200-gcc0aac99d977 #253
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
    Call Trace:
     __dump_stack lib/dump_stack.c:17 [inline]
     dump_stack+0xe3/0x195 lib/dump_stack.c:53
     p...