[ Upstream commit 6e589291 ]

A malicious/clueless root user can use EXT4_IOC_SWAP_BOOT to force a
corner casew which can lead to the file system getting corrupted.
There's no usefulness to allowing this, so just prohibit this case.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>