While Darwin implements fstatvfs, the statvfs structures f_flag
field is only defined to contain two flags, ST_RDONLY and ST_NOSUID.
So the check for MNT_LOCAL would always be false, reporting all files
as non-local.

To mitigate that, on Darwin we can just use fstatfs and check
statfs.f_flags for MNT_LOCAL.

(cherry picked from commit 132ef662)
Signed-off-by: Marvin Scholz <epirat07@gmail.com>