[Bug 266886] [FUSEFS] Handling of l_whence with fcntl(F_GETLK)

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266886

            Bug ID: 266886
           Summary: [FUSEFS] Handling of l_whence with fcntl(F_GETLK)
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: jmillikin@gmail.com

I'm not 100% sure this is a bug, but it's a difference in behavior from FUSE on
Linux, and IMO the Linux version's behavior is more correct here.

When using fcntl(F_GETLK) to check for the presence of advisory locks, it's
possible to set the flock structure's `l_whence` field to `SEEK_CUR` or
`SEEK_END` to adjust how the `l_start` and `l_end` fields are interpreted. For
example, `SEEK_CUR` means they are relative to the file's current position.

FUSE doesn't pass through `l_whence`, so the Linux kernel does the adjustment
itself when generating the FUSE request. When parsing the response, it
overwrites the flock structure's `l_whence` field to `SEEK_SET` to signal that
absolute positions are being reported. 

FreeBSD's implementation of FUSE_GETLK doesn't have this behavior, it passes
through the start and end offsets as-is in both directions. This causes FUSE
mounts to have abberent advisory lock reporting for binaries that depend on the
Linux behavior.

(all of the above applies to F_SETLK / F_SETLKW also)

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