nfs/write: Use common error handling code in nfs_lock_and_join_requests()

Add a jump target so that a bit of exception handling can be better reused
at the end of this function.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>