Btrfs: fix wrong error returned by adding a device
Btrfs: fix wrong error returned by adding a device

Reproduce:
$ mkfs.btrfs /dev/sdb7
$ mount /dev/sdb7 /mnt/btrfs -o ro
$ btrfs dev add /dev/sdb8 /mnt/btrfs
ERROR: error adding the device '/dev/sdb8' - Invalid argument

Since we mount with readonly options, and /dev/sdb7 is not a seeding one,
a readonly notification is preferred.

Signed-off-by: Liu Bo <00b627b8656913e4df3ede997ad1311ebf4b82a4@cn.fujitsu.com>
Reviewed-by: Josef Bacik <78b342861d821967b29f951b7366f5a0267e0c38@redhat.com>

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