nftw: fix use of uninitialized struct stat

nftw: fix use of uninitialized struct stat

If lstat/stat fails with EACCES, st is left uninitialized, but its
st_dev/st_ino fields are then used in several places:

* for FTW_MOUNT check (in practice typically results in a false
  positive and an early return)
* for copying to the new struct history (though the struct is not used
  afterwards since we don't recurse in this case)
* for cycle detection check (could theoretically result in a false
  positive and an early return)

To avoid adding FTW_NS checks to all these places, fix this by
zero-initializing st_dev/st_ino (which can never match an existing
dentry due to zero inode being reserved in Linux), and check for FTW_NS
only when handling FTW_MOUNT since we need two valid dentries there.