pNFS/flexfiles: Ensure we have enough buffer for layoutreturn

The flexfiles client can piggyback both layout errors and layoutstats
as part of the layoutreturn. Both these payloads can get large, with
20 layout error entries taking up about 1.2K, and 4 layoutstats entries
taking up another 1K.
This patch allows a maximum payload of 4k by allocating a full page.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>