Linux 6.1 compat: open inside tmpfile()

Linux 863f144 modified the .tmpfile interface to pass a struct file,
rather than a struct dentry, and expect the tmpfile implementation to
open inside of tmpfile().

This patch implements a configuration test that checks for this new API
and appropriately sets a HAVE_TMPFILE_DENTRY flag that tracks this old
API. Contingent on this flag, the appropriate API is implemented.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #14301
Closes #14343