btrfs: use memparse

Use memparse() instead of its own private implementation.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --cc fs/btrfs/ctree.h
Simple merge

diff --cc fs/btrfs/ioctl.c
Simple merge

diff --cc fs/btrfs/super.c
Simple merge