9p: fix ignored return value in v9fs_dir_release

retval from filemap_fdatawrite was immediately overwritten by the
following p9_fid_put: preserve any error in fdatawrite if there
was any first.

This fixes the following scan-build warning:
fs/9p/vfs_dir.c:220:4: warning: Value stored to 'retval' is never read [deadcode.DeadStores]
                        retval = filemap_fdatawrite(inode->i_mapping);
                        ^        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 89c58cb3 ("fs/9p: fix error reporting in v9fs_dir_release")
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>