Btrfs: get write access when setting the default subvolume
Btrfs: get write access when setting the default subvolume

When wen want to set the default subvolume, we must get write access, or
we will change the R/O file system.

Signed-off-by: Miao Xie <d7fac3f7b328449401fe64671e732ac315caee28@cn.fujitsu.com>
Signed-off-by: Chris Mason <a169954b4cb1a46cee25f659d3bddfebe02b5fba@fusionio.com>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas