Btrfs: add sanity checks regarding to parsing mount options

I just notice the following commands succeed:
	mount <dev> <mnt> -o thread_pool=-1

This is ridiculous, only positive thread_pool makes sense,this
patch adds sanity checks for them, and also catches the error of
ENOMEM if allocating memory fails.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>