Fix: userspace-probe: truncating binary path for SDT

Fix: userspace-probe: truncating binary path for SDT

Issue
=====
This issue was uncovered when we enabled the testing of the SDT
userspace probe instrumentation on the CI, where the paths to file are
specially long.

The reported error is:
  -    rule: ma-probe-sdt (type: kernel:uprobe, location type: SDT, location: /root/workspace/dev_gerrit_lttng-tools_rootbuild/arch/amd64/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/amd64-rootnode/test_type/base/src/lttng-tools/tests/utils/testapp/userspace-probe-sdt-binary/.libs/userspace-probe-sdt-binary:foobar:tp1)
  +    rule: ma-probe-sdt (type: kernel:uprobe, location type: SDT, location: /root/workspace/dev_gerrit_lttng-tools_rootbuild/arch/amd64/babeltrace_version/stable-2.0/build/std/conf/agents/liburcu_version/master/node/amd64-rootnode/test_type/base/src/lttng-tools/tests/utils/testapp/userspace-probe-sdt-binary/.libs/userspace-probe-s:foobar:tp1)

The important part to notice is that the path to the binary is truncated
compared to was is expected by the test case.

The problem is caused by the
`lttng_userspace_probe_location_tracepoint_create_from_payload()`
function that strdup() the path string using the wrong defined value.

Fix
===
Use LTTNG_PATH_MAX rather then LTTNG_SYMBOL_NAME_LEN to copy the binary
path.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I24cbf413baba405bf4c4b534ccbc2b18f8d5d43f