fs: exfat: Use strncpy() and bail on too long filenames

fs: exfat: Use strncpy() and bail on too long filenames

In case the filename is too long, longer than PATH_MAX - 1, it
would overflow dirs->dirname array. Add missing check and also
use strncpy() to prevent the overflow in any case.

Fixes CID 550305:  Security best practices violations  (STRING_OVERFLOW)

Signed-off-by: Marek Vasut <marex@denx.de>