Fix: handle large number of pages or subbuffers per buffer

Fix: handle large number of pages or subbuffers per buffer

Do no trigger kernel console warnings when we try to allocate too many
pages, or a too large kmalloc area for page array (within a subbuffer),
or a sub-buffer array (within a buffer).

Use vmalloc/vfree for the "pages" local variable used only during
allocation, which is an array of nr_subbuf * nr_pages_per_subbuf
pointers. This ensures we do not limit the overall buffer size due to
kmalloc limitations.

Fixes #1031

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>