fdescfs: descend through /dev/fd/N when mounted linrdlnk

Linux resolves a path below /proc/self/fd/N in the directory the
descriptor names, and linprocfs makes /proc/<pid>/fd a symlink to
/dev/fd.

fdescfs: descend through /dev/fd/N when mounted linrdlnk

Linux resolves a path below /proc/self/fd/N in the directory the
descriptor names, and linprocfs makes /proc/<pid>/fd a symlink to
/dev/fd. Under linrdlnk the fdescfs node carries only VV_READLINK,
which namei will not walk through, so such a path fails with ENOTDIR.

Return the underlying vnode from fdesc_lookup for a non-final component,
or a trailing slash, reusing the machinery the nodup option already
uses. The last component is untouched, so open("/dev/fd/N") keeps its
dup(2) semantic; a descriptor with no vnode behind it, such as a pipe,
yields ENOTDIR.

Add ATF coverage for traversal, descriptor reuse, and preservation of
last-component and mount-option semantics.

Approved by: adrian (mentor)
Reviewed by: kib, adrian
Differential Revision: https://reviews.freebsd.org/D59393

Signed-off-by: Nick Price <nprice@FreeBSD.org>

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tests/fusefs: fix the build with gcc 16

Reviewed by: asomers
MFC after: 3 days
Sponsored by: The FreeBSD Foundation