Need to happen before we allocate and then leak the xefi. Found by
coverity via an xfsprogs libxfs scan.

[djwong: This also fixes the type of the @agbno argument.]

Fixes: 7dfee17b ("xfs: validate block number being freed before adding to xefi")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>