FFS extattr: fix handling of the tail

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