Fix memory leak in mount

[trix_juniper.net]

Summary

getmntopts.c build_iovec_argf
Introduce a new variable 'sval' to hold memory allocated with strdup and to free it when unused.

Test Plan

Use clang's static analyzer, scan-build, to find the problem and later to show resolution.

[cem]

Please upload diffs with context. (Either use diff -U999999 and upload, or use the arc tool associated with Phabricator.)

[trix_juniper.net]

More surrounding context

[cem]

This change is bogus. The strdup() memory is owned by the iov structure (iov_base in build_iovec()) and needed by callers. Callers will later free the memory via free_iovec().