fs/buffer.c: fold init_buffer() into init_page_buffers()

Since commit e7600409 ("fs/buffer.c: remove unnecessary init
operation after allocating buffer_head"), there are no callers of
init_buffer() outside of init_page_buffers().  So just fold it into
init_page_buffers().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>