bcachefs: thread_with_file: fix various printf problems
Experimentally fix some problems with stdio_redirect_vprintf by creating
a MOO variant with which we can experiment. We can't do a GFP_KERNEL
allocation while holding the spinlock, and I don't like how the printf
function can silently truncate the output if memory allocation fails.
Signed-off-by: Darrick J. Wong <djwong@kernel.org> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>