xfs: cleanup xlog_get_iclog_buffer_size

We don't really need all the messy branches in the function, as it
really does three things, out of which 2 are common for all branches:

 1) set up mount point log buffer size and count values if not already
    done from mount options
 2) calculate the number of log headers
 3) set up all the values in struct xlog based on the above

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>