rtla: Enforce exact match for time unit suffixes

The parse_ns_duration() function currently uses prefix matching for
detecting time units. This approach is problematic as it silently
accepts malformed strings such as "100nsx" or "100us_invalid" by
ignoring the trailing characters, leading to potential configuration
errors.

Introduce a match_time_unit() helper that checks the suffix matches
exactly and is followed by either end-of-string or a ':' delimiter.
The ':' is needed because parse_ns_duration() is also called from
get_long_ns_after_colon() when parsing SCHED_DEADLINE priority
specifications in the format "d:runtime:period" (e.g., "d:10ms:100ms").

A plain strcmp() would reject valid deadline strings because the suffix
"ms" is followed by ":100ms", not end-of-string. Similarly,
strncmp_static() would fail because ARRAY_SIZE() includes the NUL
terminator, making it equivalent to strcmp() for this comparison.

The match_time_unit() helper solves both problems: it rejects malformed
input like "100msx" while correctly handling the colon-delimited
deadline format.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260309195040.1019085-13-wander@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>

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