fs: bounds-check direct blocks - a file over 2 KiB ate the next inode

sys_getb/sys_putb indexed inode.blocks[pos/256] with no limit at all, so a
file that grew past its pointers just kept walking: first through the 4
unused bytes at the tail of the 16-byte inode, then straight into the NEXT
inode, reading its type/nlink/size as block numbers.

It hides well. Reads and writes alias identically, so a big file can be
written and read back byte-for-byte and look fine - until something else
touches the neighbouring inode, after which the tail of the file is garbage
from whatever block those bytes now name. Found by serving a 7 KB file over
httpd: corruption began at exactly offset 3072, and only sometimes.

- NDIRECT 8 -> 12: bytes 4..15 are all block pointers now, which is what the
  runaway indexing was already doing by accident. Files go to 3 KiB, no
  on-disk layout change, no format bump.
- getb past the last direct block reports EOF; putb drops the byte like a
  full disk. A capped file beats a corrupted neighbour.

count 250 > big (7142 bytes) now stops at 3072 and its neighbours survive.

0 files changed, 0 insertions, 0 deletions