We need to check if file system size is equal to provider's size, because

We need to check if file system size is equal to provider's size, because
sysinstall(8) still bogusly puts first partition at offset 0 instead of 16,
so glabel/ufs will find file system on slice instead of partition.

Before sysinstall is fixed, we must keep this code, which means that we
wont't be able to detect UFS file systems created with 'newfs -s ...'.

PS. bsdlabel(8) creates partitions properly.

MFC after:	3 days