[Bug 199152] msdosfs writes on umount to readonly mounted fs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199152

            Bug ID: 199152
           Summary: msdosfs writes on umount to readonly mounted fs
           Product: Base System
           Version: 10.1-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: longwitz at incore.de

Created attachment 155169
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=155169&action=edit
Patch for msdosfs

In FreeBSD 10.1-STABLE #10 r279940 I see

mount -r /mydosfs
umount /mydosfs
umount: unmount of /mydosfs failed: Operation not permitted
On console: g_vfs_done():ada0p1[WRITE(offset=512, length=512)]error = 1

The next umount works.

This problem was introduced by rev 275638 to head, because in the case of a
readonly mounted msdosfs a call of msdosfs_sync() was introduced which failes
to write in msdos_fsiflush(). This happens because the flag MSDOSFS_FSIMOD is
set on the first call of msdosfs_sync().

The attached patch solves the problem for me and includes also some minor
changes necessary to build the kernel with MSDOS_DEBUG.

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