Use errors=remount-ro mount option with ext3 filesystems only

[cclerget]

Description of the Pull Request (PR):

Since 5.4.0 kernel passing errors=remount-ro with squashfs mount options returns an error, fix that by conditionally set this option for ext3 filesystems only.

This fixes or addresses the following GitHub issues:

• Fixes Unable to mount squashfs via singularity exec with kernel 5.4.0 #4801

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR and tested this PR locally with a make testall
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File

Attn: @singularity-maintainers

[jmstover]

There's a syscall.Mount in pkg/ocibundle/sif/bundle_linux.go ... Does that need modified as well?

Edit:
And in internal/pkg/runtime/engine/imgbuild/create_linux.go there's a rpcOpt.Mount that's passing errors=remount-ro ... not sure if it's relevant

[cclerget]

There's a syscall.Mount in pkg/ocibundle/sif/bundle_linux.go ... Does that need modified as well?

Edit:
And in internal/pkg/runtime/engine/imgbuild/create_linux.go there's a rpcOpt.Mount that's passing errors=remount-ro ... not sure if it's relevant

Good catch both fixed. Thanks !

[jmstover]

Does this need to merge back into master as well?