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: 8e51f908 ("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>