fusefs: fix two bugs regarding _PC_MIN_HOLE_SIZE

Background:

If a user does pathconf(_, _PC_MIN_HOLE_SIZE) on a fusefs file system,
the kernel must actually issue a FUSE_LSEEK operation in order to
determine whether the server supports it. We cache that result, so we
only have to send FUSE_LSEEK the first time that _PC_MIN_HOLE_SIZE is
requested on any given mountpoint.

Problem 1:

Unlike fpathconf, pathconf operates on files that may not be open. But
FUSE_LSEEK requires the file to be open. As described in PR 278135,
FUSE_LSEEK cannot be sent for unopened files, causing _PC_MIN_HOLE_size
to wrongly report EINVAL. We never noticed that before because the
fusefs test suite only uses fpathconf, not pathconf. Fix this bug by
opening the file if necessary.

Problem 2:

On a completely sparse file, with no data blocks at all, FUSE_LSEEK with
SEEK_DATA would fail to ENXIO. That's correct behavior, but
fuse_vnop_pathconf wrongly interpreted that as "FUSE_LSEEK not
supported". Fix the interpretation.

PR: 278135
MFC after: 1 week
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D44618