libnvpair.c: replace strstr() with strchr() for a single character

Since we're looking for a single new-line character in the haystack,
it's better (and slightly more efficient) to use strchr() instead of
strstr().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: rilysh <nightquick@proton.me>
Closes #15798