vfs: fix vfs_bio_bzero_buf() for PAGE_SIZE > block size

[chs]

Summary

The "base" argument to vfs_bio_bzero_buf() is the offset within
the buf, but when the page size is larger than the buf size
then the buf might not start at the beginning of its page.
Add the offset of the buf within the page to account for this.

Sponsored by: Netflix

[kib]

Consider an example where base == PAGE_SIZE / 2, b_offset == PAGE_SIZE / 2. In this case pgoff == PAGE_SIZE, n = 0, and each iteration of the loop seems to do nothing.

[kib]

And generally pgoff might be larger than PAGE_SIZE, in which case n become negative.