xfsprogs: fix return value of verify_set_primary_sb()

xfsprogs: fix return value of verify_set_primary_sb()

If get_sb() fails because of EOF, it will return with retval 1, which will
then be interpreted as XR_BAD_MAGIC("bad magic number") in phase1() when
warning the user.

This patch fix it by using XR_EOF here, so it would be interpreted correctly.
Also change the associated comments about the return value.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>