files: try to change fsuid only if linkat() failed

We found that linkat for "unsafe" files doesn't work in userns
if a file uid isn't equal to the currect fsuid. This issue was
fixed by changing fsuid before calling linkat. But in this
case we are not able to createa link if a target directory doesn't
have write premissions.

Starting with the 4.3 kernel, it's possible to create links of
"unsafe files":

f2ca379642d7 ("namei: permit linking with CAP_FOWNER in userns")

So we can try to call linkat() without changing fsuid and make one
more attempt with changing fsuid if the first one failed with EPERM.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>