lib: Pass the correct align offset to bitmap_find_next_zero_area_off

bitmap_find_next_zero_area_off requires everything to be given in terms
of bits. genpool no longer keeps track of everything in terms of bits
so the start address must be shifted before passing to
bitmap_find_next_zero_area_off. Without this, if the start address
is unaligned, the code will not take this into account.

Change-Id: I49e3dba51c48e2f97b03b21304e1258e83881c6b
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>