ueventd: only relabel devices if there's a delta

Since commit: 24a3cbfa
ueventd is attempting to relabel devices on calls to
make_device() when the device already exists. However, it
should only be called if the contexts for relabelfrom and
relabelto are different.

When this condition occurs, one will see denials like this:

[   14.409448] type=1400 audit(978307239.225:26): avc: denied { relabelfrom } for pid=1572 comm="ueventd" name="ttyS0" dev="tmpfs" ino=9613 scontext=u:r:ueventd:s0 tcontext=u:object_r:hci_attach_dev:s0 tclass=chr_file permissive=1
[   14.428107] type=1400 audit(978307239.225:27): avc: denied { relabelto } for pid=1572 comm="ueventd" name="ttyS0" dev="tmpfs" ino=9613 scontext=u:r:ueventd:s0 tcontext=u:object_r:hci_attach_dev:s0 tclass=chr_file permissive=1

Bug: 29106809

Change-Id: I2105b169206f0b26420f3c4b0ba5c49aeb98da92
Signed-off-by: William Roberts <william.c.roberts@intel.com>