Suspend deadlocks when trying to unregister /sys/block/sr0.

This comes from Oliver's commit 94bebf4d
"Driver core: fix race in sysfs between sysfs_remove_file() and
read()/write()".

sysfs_write_file downs buffer->sem while calling flush_write_buffer, and
flushing that particular write buffer entails downing buffer->sem in
orphan_all_buffers, resulting in the obvious self-deadlock.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>