staging: comedi: comedi_bond: use krealloc() and fix memory leak

`do_dev_config()` is called from the comedi 'attach' handler,
`bonding_attach()`.  The device private data structure contains a
dynamically allocated array of pointers to "bonded" device structures
which grows during the `do_dev_config()` call.  The length of this array
is in `devpriv->ndevs`.  It currently uses a local function `realloc()`
to allocate a new array, copy the old contents over and free the old
array.  It should be more efficient to use `krealloc()` as it may be
able to use slack space at the end of the previous array and avoid a
copy.

The old `realloc()` function always freed the old buffer which meant
that if it failed to allocate the new buffer it would lose the contents
of the old buffer.  Unfortunately, that contained pointers to more
dynamically allocated memory, leading to a memory leak.  If `krealloc()`
fails, keep the old buffer and avoid the memory leak.  The
aforementioned pointers to more dynamically allocated memory will be
cleaned up by the 'detach' handler, `bonding_detach()` which will be
called by the comedi core as a consequence of `krealloc()` failing in
`do_dev_config()`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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