[patch] backport commit=NNN mount option for reiserfs to 2.4

[Micha Feigin <michf@post.tau.ac.il>]

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I backported the commit=NNN mount option for reiserfs to 2.4 (the patch
is against 2.4.25-pre7).
This mount option allows to set the journal maximum age so that
reiserfs would play nicely with laptop-mode.
I did change the settings a bit so that setting commit=0 sets the
journals maximum age to the default value so that the value can be
switched between the default and a longer one when laptop-mode is
activated (since if I am not mistaken the current value can't be
extracted in user mode unless the reiserfs proc entry option is
compiled in and thus this can't be done at the script level).
This did take a bit of work since reiserfs stores this value on disk in
the journal header which isn't kept in memory later. Thus the value
needs to either be reread from disk when resetting to the default value
or stored in memory with the current value.
Each has its problems. I went with the first option since there seems
to be disk activity anyway during remount, and this is non-intrusive
on the current structures.
This is instead of the current implementation that ext3 uses for
laptop-mode (there is no such implementation for reiserfs in 2.4),
which after looking at the code may be a bit less intrusive but is more
of a hack (and it does tramp over the existing commit=NNN mount option
in ext3 BTW).
Any comments are welcome.

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