os: fix readdir_r buffer size

The buffer needs to be big or else we're walk all over the stack.

Backport: dumpling, cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
os: make readdir_r buffers larger

PATH_MAX isn't quite big enough.

Backport: dumpling, cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #535 from ceph/wip-readdir-r-sucks

Fix readdir_r invocation

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>