NFS: report more appropriate block size for directories.

NFS: report more appropriate block size for directories.

In glibc 2.21 (and several previous), a call to opendir() will
result in a 32K (BUFSIZ*4) buffer being allocated and passed to
getdents.

However a call to fdopendir() results in an 'fstat' request to
determine block size and a matching buffer allocated for subsequent
use with getdents.  This will typically be 1M.

The first getdents call on an NFS directory will always use
READDIR_PLUS (or NFSv4 equivalent) if available.  Subsequent getdents
calls only use this more expensive version if some 'stat' requests are
made between the getdents calls.

For this reason it is good to keep at least that first getdents call
relatively short.  When fdopendir() and readdir() is used on a large
directory, it takes approximately 32 times as long to complete as
using "opendir".  Current versions of 'find' use fdopendir() and
demonstrate this slowness.

'stat' on a directory currently returns the 'wsize'.  This number has
no meaning on directories.
Actual READDIR requests are limited to ->dtsize, which itself is
capped at 4 pages, coincidently the same as BUFSIZ*4.
So this is a meaningful number to use as the blocksize on directories,
and has the effect of making 'find' on large directories go a lot
faster.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>