nfs: fix comment to nfs_generic_pg_test which does the opposite

Please see comment to filelayout_pg_test for reference.

To: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/pagelist.c

@@ -461,7 +461,7 @@ EXPORT_SYMBOL_GPL(nfs_wait_on_request);
  * @prev: previous request in desc, or NULL
  * @req: this request
  *
- * Returns zero if @req can be coalesced into @desc, otherwise it returns
+ * Returns zero if @req cannot be coalesced into @desc, otherwise it returns
  * the size of the request.
  */
 size_t nfs_generic_pg_test(struct nfs_pageio_descriptor *desc,