FFS extattr: fix handling of the tail

[kib]

Summary

There are three issues with change that stopped truncating ea area before write, and resulted in possible zero tail in the area:

• Truncate to zero checked i_ea_len after the reference was dropped, making the last drop effectively truncate to zero length always.
• Loop to fill uio for zeroing specified too large length.
• Integrity check could trip over the tail, instead we must allow partial header or header with zero length, and chomp ea at it.

Fixup: 5e198e7646a27412c0541719f7bf1bbc0bd89223

Test Plan

Test case looks like this:

mdconfig -a -t swap -s 32m
newfs -U /dev/md0
mount /dev/md0 /mnt
touch /mnt/foo

setextattr user a aaaa /mnt/foo
setextattr user b bbbb /mnt/foo
setextattr user c cccc /mnt/foo

rmextattr user c /mnt/foo
lsextattr user /mnt/foo

getextattr user a /mnt/foo
getextattr user b /mnt/foo

[arichardson]

I can confirm that this fixes the failing tests for me: /usr/tests/usr.sbin/extattr # env TMPDIR=/root/tmp kyua test

[pho]

I ran your test scenario plus the five other extattr scenarios I already had. LGTM.

[mckusick]

Sorry for the delayed review. This change looks correct to me.