xfs: refactor backing memory allocations for buffers

Lift handling of shmem and slab backed buffers into xfs_buf_alloc_pages
and rename the result to xfs_buf_alloc_backing_mem.  This shares more
code and ensures uncached buffers can also use slab, which slightly
reduces the memory usage of growfs on 512 byte sector size file systems,
but more importantly means the allocation invariants are the same for
cached and uncached buffers.  Document these new invariants with a big
fat comment mostly stolen from a patch by Dave Chinner.

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

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