btrfs: btrfs_inherit_iflags() can be static

btrfs_new_inode() is the only consumer move it to inode.c,
from ioctl.c.

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