This commit defines a new capability called `FUSE_CAP_CACHE_SYMLINKS`.
It is off by default but you can now enable it by setting this flag in
in the `want` field of the `fuse_conn_info` structure.
When enabled, the kernel will save symlinks in its page cache,
by making use of the feature introduced in kernel 4.20:
https://github.com/torvalds/linux/commit/5571f1e65486be025f73fa6aa30fb03725d362a2
IN a bunch of comments we say 'under the terms of the GNU GPL', make
it clear this is GPLv2 (as LICENSE says).
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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