apparmor: fix error returns checks by making size a ssize_t

Currently variable size is a unsigned size_t, hence comparisons to
see if it is less than zero (for error checking) will always be
false.  Fix this by making size a ssize_t

Detected by CoverityScan, CID#1466080 ("Unsigned compared against 0")

Fixes: 8e51f9087f40 ("apparmor: Add support for attaching profiles via xattr, presence and value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/domain.c

@@ -314,7 +314,7 @@ static int aa_xattrs_match(const struct linux_binprm *bprm,
 			   struct aa_profile *profile, unsigned int state)
 {
 	int i;
-	size_t size;
+	ssize_t size;
 	struct dentry *d;
 	char *value = NULL;
 	int value_size = 0, ret = profile->xattr_count;