do_readlinkat(): import pathname only once

Take getname_flags() and putname() outside of retry loop.

Since getname_flags() is the only thing that cares about LOOKUP_EMPTY,
don't bother with setting LOOKUP_EMPTY in lookup_flags - just pass it
to getname_flags() and be done with that.

The things could be further simplified by use of cleanup.h stuff, but
let's not clutter the patch with that.

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