xfs: Fix xfs_grow_last_rtg()

The last rtg should be able to grow when the size of the last is less
than (and not equal to) sb_rgextents. xfs_growfs with realtime groups
fails without this patch. The reason is that, xfs_growfs_rtg() tries
to grow the last rt group even when the last rt group is at its
maximal size i.e, sb_rgextents. It fails with the following messages:

XFS (loop0): Internal error block >= mp->m_rsumblocks at line 253 of file fs/xfs/libxfs/xfs_rtbitmap.c. Caller xfs_rtsummary_read_buf+0x20/0x80
XFS (loop0): Corruption detected. Unmount and run xfs_repair
XFS (loop0): Internal error xfs_trans_cancel at line 976 of file fs/xfs/xfs_trans.c. Caller xfs_growfs_rt_bmblock+0x402/0x450
XFS (loop0): Corruption of in-memory data (0x8) detected at xfs_trans_cancel+0x10a/0x1f0 (fs/xfs/xfs_trans.c:977). Shutting down filesystem.
XFS (loop0): Please unmount the filesystem and rectify the problem(s)

Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>