common/rc: fix _try_scratch_mount() and _test_mount() when mount fails

common/rc: fix _try_scratch_mount() and _test_mount() when mount fails

After the recent commit 4a7b35d7a76cd9 ("common: allow to run all tests
on idmapped mounts"), some test that use _try_scratch_mount started to
fail. For example:

$ ./check btrfs/131 btrfs/220
FSTYP         -- btrfs
PLATFORM      -- Linux/x86_64 debian9 5.17.0-rc8-btrfs-next-114 (...)
MKFS_OPTIONS  -- /dev/sdc
MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1

btrfs/131 2s ... - output mismatch (see .../results//btrfs/131.out.bad)
    --- tests/btrfs/131.out	2020-06-10 19:29:03.818519162 +0100
    +++ /home/fdmanana/git/hub/xfstests/results//btrfs/131.out.bad (...)
    @@ -6,8 +6,6 @@
     Disabling free space cache and enabling free space tree
     free space tree is enabled
     Trying to mount without free space tree
    -mount failed
    -mount failed
     Mounting existing free space tree
     free space tree is enabled
    ...
    (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/131.out ...
btrfs/220 7s ... - output mismatch (see .../results//btrfs/220.out.bad)
    --- tests/btrfs/220.out	2020-10-16 23:13:46.802162554 +0100
    +++ /home/fdmanana/git/hub/xfstests/results//btrfs/220.out.bad (...)
    @@ -1,2 +1,32 @@
     QA output created by 220
    +Option fragment=invalid should fail to mount
    +umount: /home/fdmanana/btrfs-tests/scratch_1: not mounted.
    +Option nologreplay should fail to mount
    +umount: /home/fdmanana/btrfs-tests/scratch_1: not mounted.
    +Option norecovery should fail to mount
    +umount: /home/fdmanana/btrfs-tests/scratch_1: not mounted.
    ...
    (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/220.out ...
Ran: btrfs/131 btrfs/220
Failures: btrfs/131 btrfs/220
Failed 2 of 2 tests

The reason is that if _try_scratch_mount() fails to mount the filesystem,
we don't return the failure, instead we call _idmapped_mount(), which
can succeed and make _try_scratch_mount() return 0 (success). The same
happens for _test_mount(), however a quick search revealed no tests
currently relying on the return value of _test_mount().

So fix that by making _try_scratch_mount() return immediately if it gets
a mount failure. Also do the same for _test_mount().

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>