selinux: cleanup return codes in avtab_read_item()

The avtab_read_item() function tends to return -1 as a default error
code which is wrong (-1 means -EPERM).  I modified it to return
appropriate error codes which is -EINVAL or the error code from
next_entry() or insertf().

next_entry() returns -EINVAL.
insertf() is a function pointer to either avtab_insert() or
cond_insertf().
avtab_insert() returns -EINVAL, -ENOMEM, and -EEXIST.
cond_insertf() currently returns -1, but I will fix it in a later patch.

There is code in avtab_read() which translates the -1 returns from
avtab_read_item() to -EINVAL. The translation is no longer needed, so I
removed it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>