IB: Move the macro IB_UMEM_MAX_PAGE_CHUNK() to umem.c

After moving the definition of struct ib_umem_chunk from ib_verbs.h to
ib_umem.h there isn't any reason for the macro IB_UMEM_MAX_PAGE_CHUNK
to stay in ib_verbs.h.  Move the macro to umem.c, the only place where
it is used.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>