xfs: fix uninit warning in xfs_growfs_data

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>