NFS: Fallocate should use the nfs4_fattr_bitmap

Changing a sparse file could have an effect not only on the file size,
but also on the number of blocks used by the file in the underlying
filesystem. The server's cache_consistency_bitmap doesn't update the
SPACE_USED attribute, so let's switch to the nfs4_fattr_bitmap to catch
this update whenever we do an ALLOCATE or DEALLOCATE.

This patch fixes xfstests generic/568, which tests that fallocating an
unaligned range allocates all blocks touched by that range. Without this
patch, `stat` reports 0 bytes used immediately after the fallocate.
Adding a `sleep 5` to the test also catches the update, but it's better
to do so when we know something has changed.

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