NFS: Protect against 'eof page pollution'

This commit fixes the failing xfstest 'generic/363'.

When the user mmaps() an area that extends beyond the end of file, and
proceeds to write data into the folio that straddles that eof, we're
required to discard that folio data if the user calls some function that
extends the file length.

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