xprtrdma: fix up the nfs/nfs-anna merge of the Write chunk head trim

Two fixes for the same Write chunk payload corruption meet in
linux-next. In the nfs tree, commit a0db57fee35e ("xprtrdma: Fix Write
chunk payload corruption on a short reply header") trims
head[0].iov_len in rpcrdma_decode_msg(). In the nfs-anna tree,
commit 9ee534a75912 ("xprtrdma: trim head iovec when the payload
arrives via a Write chunk") trims it in rpcrdma_inline_fixup() and
reduces buflen by the same amount.

The merge resolution keeps the caller-side trim. By the time
rpcrdma_inline_fixup() runs, head[0].iov_len already equals copy_len,
so the block added by the nfs-anna commit never runs and its buflen
reduction in rq_rcv_buf and rq_private_buf is lost. The helper is also
still passed xdr_pad_size(writelist), while its kernel-doc now
describes @writelist as the number of bytes conveyed by the Write
chunk.

Drop the caller-side trim and pass the full Write chunk length, so
that rpcrdma_inline_fixup() is the only place that trims head[0] and
reduces buflen. Compute the pad with xdr_pad_size() in the helper, so
the padding from the nfs commit stays correct for lengths that are not
a multiple of four. Its rpclen calculation and empty reply check stay
in rpcrdma_decode_msg().

Fixes: 53ab189bdc14 ("Merge branch 'linux-next' of git://git.linux-nfs.org/projects/anna/linux-nfs.git")
Link: https://lore.kernel.org/all/as4aacUf2Q78FMvY@sirena.org.uk/
Signed-off-by: Sasha Levin <sashal@kernel.org>
Tested-by: Tim Menninger <tmenninger@everpuredata.com>
Reviewed-by: Tim Menninger <tmenninger@everpuredata.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>