xfs: suppress spurious uninitialised var warning in xfs_bmapi()

Initialise the xfs_bmalloca_t structure to zero to avoid uninitialised
variable warnings. This is done by zeroing the arg structure rather than
using the uninitialised_var() trick so we know for certain that the
structure is correctly initialised as xfs_bmapi is a very complex
function and it is difficult to prove warnings are spurious.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>