NFSv4.2: improve page handling for GETXATTR

XDRBUF_SPARSE_PAGES can cause problems for the RDMA transport,
and it's easy enough to allocate enough pages for the request
up front, so do that.

Also, since we've allocated the pages anyway, use the full
page aligned length for the receive buffer. This will allow
caching of valid replies that are too large for the caller,
but that still fit in the allocated pages.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>