LU-12275 osd: make osd layer always send complete pages

LU-12275 osd: make osd layer always send complete pages

In osd layer, instead of looking if we go beyong isize, just make sure
we send complete pages all the time.
Data in page beyond isize will be discared by client anyway, and it
should not be harmful to send at max PAGE_SIZE-1 more bytes for reads
at end of file.

With this new paradigm, we need to remove sanity test_246, as its sole
purpose is to actually make sure we do not send more than isize bytes
to the client.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I03dc6037a8dfa1d40d40a4b1f675e047d862d933
Reviewed-on: https://review.whamcloud.com/36238
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>