romfs: do not use mtd->get_unmapped_area directly

Remove direct usage of mtd->get_unmapped_area. Instead, just call
'mtd_get_unmapped_area()' which will return -EOPNOTSUPP if the function
is not implemented, and then test for this code.

We also translate -EOPNOTSUPP to -ENOSYS because this return code is
probably part of the kernel ABI which we do not want to break.

Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>