MFS12 r340849, r340850, r340851:

MFC r340661 (rmacklem):

r304026 added code that started statistics gathering for an operation
before the operation number (the variable called "op") was sanity checked.
This patch moves the code down to below the range sanity check for "op".

MFC r340662 (rmacklem):

nfsm_advance() would panic() when the offs argument was negative.
The code assumed that this would indicate a corrupted mbuf chain, but
it could simply be caused by bogus RPC message data.
This patch replaces the panic() with a printf() plus error return.

MFC r340663 (rmacklem):

Improve sanity checking for the dircount hint argument to
NFSv3's ReaddirPlus and NFSv4's Readdir operations. The code
checked for a zero argument, but did not check for a very large value.
This patch clips dircount at the server's maximum data size.

Approved by: re (kib)