xfs: separate log head record discovery from verification

The code that locates the log record at the head of the log is buried in
the log head verification function. This is fine when torn write
verification occurs unconditionally, but this behavior is problematic
for filesystems that might be moved across systems with different
architectures.

In preparation for separating examination of the log head for unmount
records from torn write detection, lift the record location logic out of
the log verification function and into the caller. This patch does not
change behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>