btrfs: filter invalid arg for btrfs resize

Originally following cmds will work:
	# btrfs fi resize -10A  <mnt>
	# btrfs fi resize -10Gaha <mnt>
Filter the arg by checking the return pointer of memparse.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>