NFSD: Increase the default max_block_size to 4MB

NFSD: Increase the default max_block_size to 4MB

Commit 8a81f16de64f ("NFSD: Add a "default" block size") introduced
NFSSVC_DEFBLKSIZE at 1MB, well below the 4MB NFSSVC_MAXBLKSIZE
ceiling, with the stated intent that a later change would raise the
default.

Raising the default reduces per-RPC overhead on fast networks by
amortizing header processing and scheduling costs across larger
payloads. The halving loop in nfsd_get_default_max_blksize()
constrains the returned value to 1/4096 of available RAM, so the
new 4MB default takes effect only on systems with at least 16GB of
RAM. Smaller machines continue to receive the same computed value
as before. Administrators can still override the computed value
through /proc/fs/nfsd/max_block_size.

On systems where the new default takes effect,
svc_sock_setbufsize() sizes each service socket's send and receive
buffers as nreqs * max_mesg * 2. Quadrupling max_mesg therefore
quadruples the per-socket buffer reservation at a fixed thread
count, which operators tuning large thread pools should account
for.

Note well: Your NFS client implementation must support large read
and write size settings to benefit from this change.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>