Btrfs: restore restriper state on all mounts

Fix a bug that triggered asserts in btrfs_balance() in both normal and
resume modes -- restriper state was not properly restored on read-only
mounts.  This factors out resuming code from btrfs_restore_balance(),
which is now also called earlier in the mount sequence to avoid the
problem of some early writes getting the old profile.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>