xfs: use xfs_buf_alloc_pages for uncached buffers

Use the newly factored out page allocation code. This adds
automatic buffer zeroing for non-read uncached buffers.

This also allows us to greatly simply the error handling in
xfs_buf_get_uncached(). Because xfs_buf_alloc_pages() cleans up
partial allocation failure, we can just call xfs_buf_free() in all
error cases now to clean up after failures.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>