xfs: always init bma in xfs_bmapi_write

Always init the tp/ip fields of bma in xfs_bmapi_write so that the
bmapi_finish at the bottom never trips over null transaction or inode
pointers.

Coverity-id: 1443964
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>