generic_file_write()'s rlimit checks are preventing writes to large
offsets into blockdevs:

# ulimit -f 10000
# dd if=/dev/zero of=/dev/sde5 bs=1k count=1 seek=1000000
zsh: file size limit exceeded

So don't apply that check if it's a blockdev.

The patch also caches the S_ISBLK result in a local.