ubi-utils: use 'stat(2)' instead of 'lstat(2)'

ubi-utils: use 'stat(2)' instead of 'lstat(2)'

The UBI tools refuse to work with symlinks like
'/dev/ubi/bootloader -> ../ubi0_2' because they use 'lstat(2)' and get
information about the symlink but not about the device.  This is
unwanted and fixed by this patch.

This patch renames 'struct stat stat' variables to 'st' to avoid
compilation errors.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>