xfs: split xfs_buf_rele for cached vs uncached buffers

xfs_buf_rele is a bit confusing because it mixes up handling of normal
cached and the special uncached buffers without much explanation.
Split the handling into two different helpers, and use a clearly named
helper that checks the hash key to distinguish the two cases instead
of checking the pag pointer.

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