ceph: fix ENOENT logic in striped_read

Getting ENOENT is equivalent to reading 0 bytes.  Make that correction
before setting up the hit_stripe and was_short flags.

Fixes the following case:
 dd if=/dev/zero of=/mnt/fs_depot/dd3 bs=1 seek=1048576 count=0
 dd if=/mnt/fs_depot/dd3 of=/root/ddout1 skip=8 bs=500 count=2 iflag=direct

Reported-by: Henry C Chang <henry.cy.chang@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>