[XFS] Fix realtime subvolume expansion, a porting bug b0rked it. Coverity

made me look at this code (bug id #344). We only return with
XFS_ERROR(EINVAL) if mp->m_rtdev_targp is valid and pass it otherwise to
xfs_read_buf() where some function calls later it gets dereferenced by an
assert.

SGI-PV: 954266
SGI-Modid: xfs-linux-melb:xfs-kern:26363a

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Nathan Scott <nathans@sgi.com>

fs/xfs/xfs_rtalloc.c

@@ -1929,7 +1929,7 @@ xfs_growfs_rt(
 	/*
 	 * Initial error checking.
 	 */
-	if (mp->m_rtdev_targp || mp->m_rbmip == NULL ||
+	if (mp->m_rtdev_targp == NULL || mp->m_rbmip == NULL ||
 	    (nrblocks = in->newblocks) <= sbp->sb_rblocks ||
 	    (sbp->sb_rblocks && (in->extsize != sbp->sb_rextsize)))
 		return XFS_ERROR(EINVAL);