9p: Add mount option for negative dentry cache retention

9p: Add mount option for negative dentry cache retention

Introduce a new mount option, negtimeout, for v9fs that allows users
to specify how long negative dentries are retained in the cache. The
retention time can be set in milliseconds (e.g. negtimeout=10000 for
a 10secs retention time) or a negative value (e.g. negtimeout=-1) to
keep negative entries until the buffer cache management removes them.

For consistency reasons, this option should only be used in exclusive
or read-only mount scenarios, aligning with the cache=loose usage.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Message-ID: <b2d66500aa5a2f6540347c4aa46a4be10dd01bc6.1779355927.git.repk@triplefau.lt>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>