efivarfs: Free s_fs_info on unmount

[ Upstream commit 547713d5 ]

Now that we allocate a s_fs_info struct on fs context creation, we
should ensure that we free it again when the superblock goes away.

Fixes: 5329aa51 ("efivarfs: Add uid/gid mount options")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>