comedi: don't use mutex for COMEDI_BUFINFO ioctl

comedi: don't use mutex for COMEDI_BUFINFO ioctl

The main mutex in a comedi device can get held for quite a while when
processing comedi instructions, so for performance reasons, the "read",
"write", and "poll" file operations do not use it; they use the
`attach_lock` rwsemaphore to protect against the comedi device becoming
detached at an inopportune moment.  As an alternative to using the
"read" and "write" operations, user-space can mmap the data buffer and
use the `COMEDI_BUFINFO` ioctl to manage data transfer through the
buffer.  However, the "ioctl" file handler currently locks the main
mutex for all ioctl commands.  Make the handling of the `COMEDI_BUFINFO`
an exception, using the `attach_lock` rwsemaphore during normal
operation.  However, before it calls `do_become_nonbusy()` at the end of
acquisition, it does need to lock the main mutex, but it needs to unlock
the `attach_lock` rwsemaphore first to avoid deadlock.  After locking
the main mutex, it needs to check that it is still in a suitable state
to become non-busy, because things may have changed while unlocked.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20251205131332.16672-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>