Quiet down this gcc warning:
fs/xfs/xfs_fsops.c: In function ‘xfs_growfs_data’:
fs/xfs/xfs_fsops.c:219:21: error: ‘lastag_extended’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  219 |                 if (lastag_extended) {
      |                     ^~~~~~~~~~~~~~~
fs/xfs/xfs_fsops.c:100:33: note: ‘lastag_extended’ was declared here
  100 |         bool                    lastag_extended;
      |                                 ^~~~~~~~~~~~~~~
By setting its value explicitly.  From code analysis I don't think this
is a real problem, but I have better things to do than analyse this
closely.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
        xfs_agnumber_t          nagimax = 0;
        xfs_rfsblock_t          nb, nb_div, nb_mod;
        int64_t                 delta;
-       bool                    lastag_extended;
+       bool                    lastag_extended = false;
        xfs_agnumber_t          oagcount;
        struct xfs_trans        *tp;
        struct aghdr_init_data  id = {};