[Bug 210316] panic after trying to r/w mount msdosfs on write protected media

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210316

--- Comment #6 from Damjan Jovanovic <damjan.jov at gmail.com> ---
Repeating that test with UFS (newfs -U -j) also freezes the system, so sadly
msdosfs is not the only filesystem affected.

Using the ext2fs driver on a read-only EXT3 filesystem however works, printing
some errors in dmesg:

g_vfs_done():ggate0[WRITE(offset=1024, length=1024)]error = 1
g_vfs_done():ggate0[WRITE(offset=1024, length=1024)]error = 1
g_vfs_done():ggate0[WRITE(offset=2048, length=1024)]error = 1
g_vfs_done():ggate0[WRITE(offset=268288, length=1024)]error = 1
g_vfs_done():ggate0[WRITE(offset=268288, length=1024)]error = 1

but it mounts successfully, denies writes with EPERM, and doesn't crash.
Unmounting however fails, with this ominous error:

fsync: giving up on dirty (error = 1) 0xfffff801df6a45a0: tag devfs, type VCHR
    usecount 1, writecount 0, refcount 7 rdev 0xfffff800d1092800
    flags (VI_ACTIVE)
    v_object 0xfffff80146eb3900 ref 0 pages 2 cleanbuf 1 dirtybuf 4
    lock type devfs: EXCL by thread 0xfffff800d1017000 (pid 1494, umount, tid
100515)
        dev ggate0

but still no crash.

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