Handle errors from background write of the cylinder group blocks.

First, on the write error, bufdone() call from ffs_backgroundwrite()
panics because pbrelvp() cleared bp->b_bufobj, while brelse() would
try to re-dirty the copy of the cg buffer. Handle this by setting
B_INVAL for the case of BIO_ERROR.

Second, we must re-dirty the real buffer containing the cylinder group
block data when background write failed. Real cg buffer was already
marked clean in ffs_bufwrite(). After the BV_BKGRDINPROG flag is
cleared on the real cg buffer in ffs_backgroundwrite(), buffer scan
may reuse the buffer at any moment. The result is lost write, and if
the write error was only transient, we get corrupted bitmaps.

We cannot re-dirty the original cg buffer in the
ffs_backgroundwritedone(), since the context is not sleepable,
preventing us from sleeping for origbp' lock. Add BV_BKGDERR flag
(protected by the buffer object lock), which is converted into delayed
write by brelse(), bqrelse() and buffer scan.

In collaboration with: Conrad Meyer <cse.cem@gmail.com>
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation (kib),

	  EMC/Isilon storage division (Conrad)

MFC after: 2 weeks