dir: check DNew return code

dir: check DNew return code

Commit 0284e65f97861e888d95576f22a93cd681813c39 'dir: Explicitly state
buffer locations for data' changed DNew and DRead to return a return
code.  However, the callers of DNew were not modified to check the new
return code.  (This commit applied only to the implementations dealing
with AFS directories, in afs/afs_buffer.c and dir/dir.c.  The ubik
implmentations of DNew and DRead, dealing with ubik databases, were not
modified.)

Modify all (non-ubik) callers of DNew to check the return code.  In
addition, modify code as needed so return codes are properly propagated
to the callers.

While here, add Doxygen comments for AddPage and FindBlobs.

Change-Id: Iabde6499745dd351f3fcda73c9f52c440a36490e
Reviewed-on: https://gerrit.openafs.org/13801
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>