fs_context: drop the unused lsm_flags member

This isn't ever used by VFS now, and it couldn't even work. Any FS that
uses the SECURITY_LSM_NATIVE_LABELS flag needs to also process the
value returned back from the LSM, so it needs to do its
security_sb_set_mnt_opts() call on its own anyway.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>