d_path: saner calling conventions for __dentry_path()

1) lift NUL-termination into the callers
2) pass pointer to the end of buffer instead of that to beginning.

(1) allows to simplify dentry_path() - we don't need to play silly
games with restoring the leading / of "//deleted" after __dentry_path()
would've overwritten it with NUL.

We also do not need to check if (either) prepend() in there fails -
if the buffer is not large enough, we'll end with negative buflen
after prepend() and __dentry_path() will return the right value
(ERR_PTR(-ENAMETOOLONG)) just fine.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>