LU-1249 debug: Auto correct improper debug buffer size setting

LU-1249 debug: Auto correct improper debug buffer size setting

Use the minimum required value when the debug buffer size setting
value is too small, and use the maximum acceptable value when it is
too large.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I89def7762f2ec9da3a25d28f7ffa9aede390eb85
Reviewed-on: http://review.whamcloud.com/2489
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>