xfs: don't use xfs_trans_reserve in xfs_trans_reserve_more

xfs_trans_reserve_more just tries to allocate additional blocks and/or
rtextents and is otherwise unrelated to the transaction reservation
logic.  Open code the block and rtextent reservation in
xfs_trans_reserve_more to prepare for simplifying xfs_trans_reserve.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>