net: http: Rename body_start to body_frag_start

Following #42026, the body_start pointer now points to the
start of the body fragment in the recv_buffer as long as there
is body in it, either entirely or partially.

Rename the body_start to body_frag_start to better reflect
what it represents.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>