UBIFS: remove mst_mutex

The 'mst_mutex' is not needed since because 'ubifs_write_master()' is only
called on the mount path and commit path. The mount path is sequential and
there is no parallelism, and the commit path is also serialized - there is only
one commit going on at a time.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>