filter: handle mount errors from CONFIG_BLK_DEV_WRITE_MOUNTED=y

filter: handle mount errors from CONFIG_BLK_DEV_WRITE_MOUNTED=y

Kernels post 6.x may have CONFIG_BLK_DEV_WRITE_MOUNTED=y which
prevents mount from opening the block device on a mounted
filesystem. This results in an error such as:

mount: <dev>: Can't open blockdev

which is not the error that callers of _filter_error_mount() are
looking for. It is, however, a direct result of the test trying
to mount an alreayd mounted filesystem, so it is reflecting the same
error case. Hence this mismatch in errors should not fail the test.

Catch this mount error and convert it to the expected
"already mounted" error for the tests that exercise this behaviour.

There is also a minor test change here to push mount failure
information to $seqres.full in the cases where mount errors occur.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>