nfs: fix error handling in lock_and_join_requests

This fixes handling of errors from nfs_page_group_lock in
nfs_lock_and_join_requests.  It now releases the inode lock and the
reference to the head request.

Reported-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Reviewed-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>