io: Fix ftw internal realloc buffer (BZ #28126)

The 106ff085 did not take in consideration the buffer might be
reallocated if the total path is larger than PATH_MAX.  The realloc
uses 'dirbuf', where 'dirstreams' is the allocated buffer.

Checked on x86_64-linux-gnu.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>