osd: Fix decode for some extent cache reads.

osd: Fix decode for some extent cache reads.

The extent cache in EC can cause the backend to perform some surprising reads. Some
of the patterns were discovered in test that caused the decode to attempt to
decode more data than was anticipated during the read planning, leading to an
assert. This simple fix reduces the scope of the decode to the minimum.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
(cherry picked from commit 2ab45a22397112916bbcdb82adb85f99599e03c0)