ofpbuf: Correct comments for ofpbuf_tail and ofpbuf_end().

The return type of ofpbuf_tail() and ofpbuf_end() is pointer, not byte.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ofpbuf: Correct comment for ofpbuf_reserve_with_tailroom().

Correct the old comment like ofpbuf_reserve() to describe the expected
behavior of ofpbuf_reserve_with_tailroom().

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>