xfs: look at m_frextents in xfs_iomap_prealloc_size for RT allocations

Add a check for files on the RT subvolume and use m_frextents instead
of m_fdblocks to adjust the preallocation size.

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