gfs2: convert simple_str to kstr

-Remove obsolete simple_str functions.
-Return error code when kstr failed.
-This patch also calls functions corresponding to destination type.

Thanks to Alexey Dobriyan for suggesting improvements in
block_store() and wdack_store()

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>