btrfs-progs: mkfs/rootdir: Don't follow symbolic link when calcuating size

btrfs-progs: mkfs/rootdir: Don't follow symbolic link when calcuating size

[BUG]
If we have a symbolic link in rootdir pointing to non-existing location,
mkfs.btrfs --rootdir will just fail:
------
$ mkfs.btrfs  -f --rootdir /tmp/rootdir/ /dev/data/btrfs
btrfs-progs v4.15.1
See http://btrfs.wiki.kernel.org for more information.

ERROR: ftw subdir walk of /tmp/rootdir/ failed: No such file or directory
------

[CAUSE]
Commit 599a0abed564 ("btrfs-progs: mkfs/rootdir: Use over-reserve method
to make size estimate easier") add extra ftw walk to estimate the
filesystem size.

Such default ftw walk will follow symbolic link and gives ENOENT error.

[FIX]
Use nftw() to specify FTW_PHYS so we won't follow symbolic link for size
calculation.

Issue: #109
Reported-by: Alexander Kanavin <alexander.kanavin@intel.com>
Fixes: 599a0abed564 ("btrfs-progs: mkfs/rootdir: Use over-reserve method to make size estimate easier")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>