qnx4: Use get_directory_fname() in qnx4_match()

Use the new common directory entry name accessor helper to avoid
confusing the compiler about over-running the file name buffer. Avoids
false positive buffer overflow warning:

[ 4849.636861] detected buffer overflow in strlen
[ 4849.636897] ------------[ cut here ]------------
[ 4849.636902] kernel BUG at lib/string.c:1165!
...
[ 4849.637047] Call Trace:
...
[ 4849.637251]  qnx4_find_entry.cold+0xc/0x18 [qnx4]
[ 4849.637264]  qnx4_lookup+0x3c/0xa0 [qnx4]

Reported-by: Ronald Monthero <debug.penguin32@gmail.com>
Closes: https://lore.kernel.org/lkml/20231112095353.579855-1-debug.penguin32@gmail.com/
Acked-by: Anders Larsen <al@alarsen.net>
Link: https://lore.kernel.org/r/20231130205120.3642477-2-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>