Fix size of symlinks and enable kernel symlink cache

[nbdd0121]

Description

Symlink inodes are expected to have size equal to the length of its target. This is defined in POSIX spec.

FUSE's symlink cache also expects this, so fixing this allows the enabling of the symlink cache. Note that since symlink targets are not mutable, the cache is always valid while the inode is valid, so there is no need for any TTL or invalidation logic.

This is expected to bring significant time saving for workloads that make use of symlinks (especially for a directory symlink, since all ops inside the symlink directory would require reading the symlink at least once).

Link to the issue in case of a bug fix.

Fix #2273

Testing details

1. Manual:
   • Mount a bucket on two mount points (with separate GCSFuse processes)
   • Create a symlink in mount1
   • Stat and see its size is correct and point to the correct target in mount2.
   • Remove and recreate the symlink in mount1 to a different target
   • Stat and see its size is correct and point to the correct target in mount2.
2. Unit tests - NA
3. Integration tests - I didn't add a test because I'm unfamiliar with this project (and go in general)'s test setup and it seems that the test infra is being reworked on.

[nbdd0121]

This improves perf by 10% in our workload on top of #2269.

[raj-prince]

Thanks @nbdd0121 for the contribution!!

Changes look good to me, but requires a couple of changes to make it mergeable.

(a) We need to guard the symlink caching under a flag like, --experimental-enable-symlink-cache (by default disabled). Keeping this experimental as we haven't performed all the testing.
(b) Could you please add unit test corresponding to size changes - here?
(c) If possible, could you please write the simple composite test for both true and false value of --experimental-enable-symlink-cache? You may refer this sample PR: #2036

Note: Adding an experimental flag would be trivial if you do this after 1 week. We are making the changes to simplify those path.

[raj-prince]

@nbdd0121 Please find the steps to add a new flag, here - https://github.com/GoogleCloudPlatform/gcsfuse/blob/master/docs/dev_guide.md#adding-a-new-param-in-gcsfuse

[nbdd0121]

Tried to write a test but adding the test to symlink_test.go doesn't seem trivial to me. The test that exists there don't use NewSymlinkInode. I tried to just use inode.NewSymLinkInode but then I can't call inode.Attribute because I don't have a ctx?

(My experience is mostly with C and Rust and I have no experience with Go at all.)