dm era: fix out-of-bounds memory access for non-zero start sector

In the Linux kernel, the following vulnerability has been resolved: dm era: fix out-of-bounds memory access for non-zero start sector dm-era tracks writes in target-relative blocks, but era_map() calculates the writeset block before applying the target offset. Tables with a non-zero start sector can therefore pass an absolute mapped-device block to metadata_current_marked(). If the absolute block is beyond the current writeset size, writeset_marked() tests past the end of the in-core bitset. KASAN reports this as a vmalloc-out-of-bounds access. Apply the target offset before calculating the era block so writeset lookups use the target-relative block number.