Ensure that files with no allocated blocks are trimmed to zero length.

UFS does not allow files to end with a hole; it requires that the
last block of a file be allocated. As fsck_ffs(8) initially scans
each allocated inode, it tracks the last allocated block in the
inode. It then checks that the inode's size falls in the last
allocated block. If the last allocated block falls before the size,
a `file size beyond end of allocated file' warning is issued and
the file is shortened to reference the last allocated block (to avoid
having it reference a hole at its end). If the last allocated block
falls after the size, a `partially truncated file' warning is issued
and all blocks following the block referenced by the size are freed.

Because of an incorrect unsigned comparison, this test was failing
to handle files with no allocated blocks but non-zero size (which
should have had their size reset to zero). Once that was fixed the
test started incorrectly complaining about short symbolic links
that place the link path in the inode rather than in a disk block.
Because these symbolic links have a non-zero size, but no allocated
blocks, fsck_ffs wanted to zero out their size. This patch has to
detect and avoid changing the size of such symbolic links.

Reported by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 1 week
Sponsored by: Netflix