FFS extattr: fix handling of the tail

There are three issues with change that stopped truncating ea area before
write, and resulted in possible zero tail in the ea 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, that triggered assert in normal situation.
• Integrity check could trip over the tail, instead we must allow partial header or header with zero length, and clamp ea image in memory at it.

Reported by: arichardson
Tested by: arichardson, pho
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Fixup: 5e198e7646a27412c0541719f7bf1bbc0bd89223
Differential Revision: https://reviews.freebsd.org/D28999