dm-bufio: Ensure reserved buffer is also allocated from get_free_pages pool

In cases where alloc_buffer fails to allocate there is a possibility for
a reserved buffer to be used. Currently the reserved buffer is allocated
from a vmalloc region and later in dm-verity hashing, the use of vmalloc'd
buffer results in a panic in BUG_ON(!virt_addr_valid(buf)) as the operation
is invalid. This change ensures that the reserved buffer is allocated from
a non-vmalloc'd region.

Change-Id: I401787bcc3cbf9c3837d3c7d569774dcce04b6c6
Signed-off-by: Sridhar Parasuram <sridhar@codeaurora.org>