bcachefs: thread_with_stdio: eliminate double buffering

The output buffer lock has to be a spinlock so that we can write to it
from interrupt context, so we can't use a direct copy_to_user; this
switches thread_with_file_read() to use fault_in_writeable() and
copy_to_user_nofault(), similar to how thread_with_file_write() works.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>