btrfs: move device->name RCU allocation and assign to btrfs_alloc_device()

btrfs: move device->name RCU allocation and assign to btrfs_alloc_device()

There is a repeating code section in the parent function after calling
btrfs_alloc_device(), as below:

      name = rcu_string_strdup(path, GFP_...);
      if (!name) {
              btrfs_free_device(device);
              return ERR_PTR(-ENOMEM);
      }
      rcu_assign_pointer(device->name, name);

Except in add_missing_dev() for obvious reasons.

This patch consolidates that repeating code into the btrfs_alloc_device()
itself so that the parent function doesn't have to duplicate code.
This consolidation also helps to review issues regarding RCU lock
violation with device->name.

Parent function device_list_add() and add_missing_dev() use GFP_NOFS for
the allocation, whereas the rest of the parent functions use GFP_KERNEL,
so bring the NOFS allocation context using memalloc_nofs_save() in the
function device_list_add() and add_missing_dev() is already doing it.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>