xfs: take m_growlock when running growfsrt

Take the grow lock when we're expanding the realtime volume, like we do
for the other growfs calls.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>