mountd: fix is_subdirectory to understand '/'

mountd: fix is_subdirectory to understand '/'

The is_subdirectory() function checks if a given 'child' is a
subdirectory of the given 'parent'.  However it always fails
if 'parent' == "/" (because 'child' doesn't begin with 'parent'
followed by "/").

So change is_subdirectory() to special-case "/".

subexport() also tests if one directory is a subdirectory of the
other, and contains the same bug.  So change it to use
is_subdirectory().

Finally, move is_subdirectory() and related path_matches() and
export_matches() earlier in the file to avoid a forward-reference.

This patch fixes a bug wherein if you export "/" with 'crossmnt', the
crossmnt flag is ineffective and you can only access the root
filesystem, not any descendants.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>