BUILD: buffers: keep b_getblk_nc() and b_peek_varint() in buf.h

BUILD: buffers: keep b_getblk_nc() and b_peek_varint() in buf.h

Some large functions were moved to buf.c by commit ac66df4e2 ("REORG:
buffers: move some of the heavy functions from buf.h to buf.c"). However,
as found by Amaury, haring doesn't build anymore. Upon close inspection,
b_getblk_nc() isn't that big since it's very much inlinable, and a part
of its apparently large size comes from the BUG_ON_HOT() that were
implemented. Regarding b_peek_varint(), it doesn't have any dependency
and is used only at 4 places in the DNS code, so its loop will not have
big impacts, and the rest around can be optimised away by the compiler
so it remains relevant to keep it inlined. Also it can serve as a base
to deduplicate the code in b_get_varint().

No backport needed.