libmount: fix access() utab write test

libmount: fix access() utab write test

The commit c08396c7691e1e6a04b6b45892e7e4612ceed8d7 replaces
open(O_CREATE) with ecaccess(). Unfortunately, another code depends on
the original behavior.

* let's make utab when really necessary rather than in the try_write() test

* __mnt_new_table_from_file() returns NULL if tab-file does not
 exists. This is incorrect for tab_update.c stuff. We need empty table
 in this case.

* we can check /run/mount/ directory for write access if
  eaccess(filename) return ENOENT (because file does not exist)

Signed-off-by: Karel Zak <kzak@redhat.com>