Do not brelse() the buffer unconditionally with BIO_ERROR set if
uiomove() failed. The brelse() treats most buffers with BIO_ERROR as
B_INVAL, dropping their content. Instead, if the write request
covered the whole buffer, remember the cached state and brelse() with
BIO_ERROR set only if the buffer was not cached previously.
Update the buffer dirtyoff/dirtyend based on the progress recorded by
uiomove() in passed struct uio, even in the presence of
error. Otherwise, usermode could see changed data in the backed pages,
but later the buffer is destroyed without write-back.
If uiomove() failed for IO_UNIT request, try to truncate the vnode
back to the pre-write state, and rewind the progress in passed uio
accordingly, following the FFS behaviour.
Reviewed by: rmacklem (some time ago)
Tested by: pho
MFC after: 1 month
bfb68a9e43