powerpc/xmon: Use KSYM_NAME_LEN in array size

kallsyms_lookup() which in turn calls kallsyms_lookup_buildid() writes
to index "KSYM_NAME_LEN - 1".

Thus the array passed as namebuf to kallsyms_lookup() should be
KSYM_NAME_LEN in size.

In xmon.c the array was defined to be "128" bytes directly, without
using KSYM_NAME_LEN. Commit b8a94bfb ("kallsyms: increase maximum
kernel symbol length to 512") changed the value to 512, but missed
updating the xmon code.

Fixes: b8a94bfb ("kallsyms: increase maximum kernel symbol length to 512")
Cc: stable@vger.kernel.org # v6.1+
Co-developed-by: Onkarnath <onkarnath.1@samsung.com>
Signed-off-by: Onkarnath <onkarnath.1@samsung.com>
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
[mpe: Tweak change log wording and fix commit reference]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230529111337.352990-2-maninder1.s@samsung.com