mount: Add helper for path length calculations

The path lenght is zero for the "/" one and strlen(path)
for all the others. This is done so to make it possible
to use this length to get tail-paths: if path_1 starts
with path_2 and both are absolute, then

   path_1 + path_length(path_2)

would give the tail of the tail of path_1 relative to
path_2 even if the path_2 is just "/".

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