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>
(cherry picked from commit 2df66d9fa214e90eb5141df4d5755b57e8ba9413)

Conflicts:

	src/os/BtrfsFileStoreBackend.cc
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>
(cherry picked from commit 99a2ff7da99f8cf70976f05d4fe7aa28dd7afae5)