sparc: Add support for seek and shorter read to /dev/mdesc

/dev/mdesc on Linux does not support reading arbitrary number
of bytes and seeking while /dev/mdesc on Solaris does. This
causes tools that work on Solaris to break on Linux. This patch
adds these two capabilities to /dev/mdesc.

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>