[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>