p9fs: Fix creating files with restrictive permissions

When a file is created via p9fs with restrictive permissions (like 000),
the 9P TCREATE request successfully creates and natively opens the file,
returning an open, writable file descriptor. Previously, p9fs would
attempt a subsequent TOPEN. That TOPEN would fail with EACCES due to the
restrictive mode, leaving a 0-byte file and causing operations like 'mv'
to abort.

We now preserve the writable descriptor returned by TCREATE so that the
subsequent VOP_OPEN can use it directly, avoiding the failing TOPEN.
Additionally, p9fs_compatible_mode now appropriately isolates the base
access intent when matching fids, preventing extended flags from
breaking the match.

A test case for this behavior has been submitted to pjdfstest:
https://github.com/pjd/pjdfstest/pull/87

Resolves: https://github.com/CTSRD-CHERI/cheribsd/issues/2617

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56494