do_last(): simplify the liveness analysis past finish_open_created

Don't mess with got_write there - it is guaranteed to be false on
entry and it will be set true if and only if we decide to go for
truncation and manage to get write access for that.

Don't carry acc_mode through the entire thing - it's only used
in that part.  And don't bother with gotos in there - compiler is
quite capable of optimizing that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>