netmap: Remove duplicate code

[markj]

Summary

This duplicates code in netmap_get_na() which looks up the memory
allocator specified by nr_mem_id, so let's deduplicate.

No functional change intended.

[vmaffione]

I guess you mean that the same code is present inside netmap_get_na which is call right after.
However, with this change the behaviour would not be equivalent, because netmap_mem_find grabs a reference to a netmap memory allocator.
So before this patch the reference is kept along the whole netmap_ioctl function, whereas with the patch the reference scope drops to the duration o the netmap_get_na call.

[markj]

In D46665#1064133, @vmaffione wrote:

I guess you mean that the same code is present inside netmap_get_na which is call right after.
However, with this change the behaviour would not be equivalent, because netmap_mem_find grabs a reference to a netmap memory allocator.
So before this patch the reference is kept along the whole netmap_ioctl function, whereas with the patch the reference scope drops to the duration o the netmap_get_na call.

I see now, apologies for the bogus patch. I thought I had tested the req->nr_mem_id != 0 case but my test program was wrong.