Only clear TDP_NERRNO when needed

[trasz]

Summary

Only clear TDP_NERRNO when needed, ie when it's previously been set.

[mjg]

this adds a branch which likely pessimizes the code

[trasz]

In D26612#592862, @mjg wrote:

this adds a branch which likely pessimizes the code

The branch is already there, you had to check td_pflags anyway. Not to mention that previously the branch was statically mispredicted (the common case is for (td_pflags & TDP_NERRNO) to be 0.