md/md-bitmap: fix incorrect usage for sb_index
In the Linux kernel, the following vulnerability has been resolved:

md/md-bitmap: fix incorrect usage for sb_index

Commit d7038f951828 ("md-bitmap: don't use ->index for pages backing the
bitmap file") removed page->index from bitmap code, but left wrong code
logic for clustered-md. current code never set slot offset for cluster
nodes, will sometimes cause crash in clustered env.

Call trace (partly):
 md_bitmap_file_set_bit+0x110/0x1d8 [md_mod]
 md_bitmap_startwrite+0x13c/0x240 [md_mod]
 raid1_make_request+0x6b0/0x1c08 [raid1]
 md_handle_request+0x1dc/0x368 [md_mod]
 md_submit_bio+0x80/0xf8 [md_mod]
 __submit_bio+0x178/0x300
 submit_bio_noacct_nocheck+0x11c/0x338
 submit_bio_noacct+0x134/0x614
 submit_bio+0x28/0xdc
 submit_bh_wbc+0x130/0x1cc
 submit_bh+0x1c/0x28