Clean-up: lttng-ctl: strnlen out of bounds access

Clean-up: lttng-ctl: strnlen out of bounds access

gcc 11.2 produces the following warning. The lttng_strncpy helper
assumes that 'src' is a null terminated string. As such, the use of a
string literal (of size 37) in this specific example is correct as
strnlen will not read beyond the null terminator.

Replacing strnlen by strlen eliminates this warning. strnlen was used to
short-circuit the source length check when it was larger than the
destination. This optimization is unlikely to matter. Pascal-style
strings should be used when string length computations are expected to
be prohibitively expensive.

In file included from ../../../src/common/macros.h:15,
                 from ../../../include/lttng/health-internal.h:18,
                 from lttng-ctl-health.cpp:19:
In function 'size_t lttng_strnlen(const char*, size_t)',
    inlined from 'int lttng_strncpy(char*, const char*, size_t)' at ../../../src/common/macros.h:123:19,
    inlined from 'int set_health_socket_path(lttng_health*, int)' at lttng-ctl-health.cpp:198:22,
    inlined from 'int lttng_health_query(lttng_health*)' at lttng-ctl-health.cpp:319:30:
../../../src/common/compat/string.h:19:23: warning: 'size_t strnlen(const char*, size_t)' specified bound 4096 may exceed source size 37 [-Wstringop-overread]
   19 |         return strnlen(str, max);
      |                ~~~~~~~^~~~~~~~~~

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I290109433fcae7073321f1b48ecfbb2ec6e4ad26