d_path: introduce struct prepend_buffer

        We've a lot of places where we have pairs of form (pointer to end
of buffer, amount of space left in front of that).  These sit in pairs of
variables located next to each other and usually passed by reference.
Turn those into instances of new type (struct prepend_buffer) and pass
reference to the pair instead of pairs of references to its fields.

Declared and initialized by DECLARE_BUFFER(name, buf, buflen).

extract_string(prepend_buffer) returns the buffer contents if
no overflow has happened, ERR_PTR(ENAMETOOLONG) otherwise.
All places where we used to have that boilerplate converted to use
of that helper.

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