xfs: clean up buffer allocation in xlog_do_recovery_pass

Merge the initial xlog_alloc_buffer calls, and pass the variable
designating the length that is initialized to 1 above instead of passing
the open coded 1 directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>