Btrfs: refactor error handling to drop inode in btrfs_create()

Refactor it by checking whether the inode has been created and needs to be
dropped (drop_inode_on_err) and also if the err variable is set. That way the
variable doesn't need to be set on each and every error handling block.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>