dm integrity: fix double free on memory allocation failure

If the statement "recalc_tags = kvmalloc(recalc_tags_size, GFP_NOIO);"
fails, we call "vfree(recalc_buffer)" and we jump to the label "oom".

If the condition "recalc_sectors >= 1U << ic->sb->log2_sectors_per_block"
is false, we jump to the label "free_ret" and call "vfree(recalc_buffer)"
again, on an already released memory block.

Fix the bug by setting "recalc_buffer = NULL" after freeing it.

Fixes: da8b4fc1 ("dm integrity: only allocate recalculate buffer when needed")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>