src.ctf.fs: don't call ds_file_munmap on failure in ds_file_mmap_next

src.ctf.fs: don't call ds_file_munmap on failure in ds_file_mmap_next

It seems useless to call ds_file_munmap here.  We can reach the error
label either because:

- the previous call to ds_file_munmap has failed, in which case there's
  no point in retrying
- the mmap call returned MAP_FAILED, in which case there's no point un
  unmapping

Change-Id: Ia11716a967047d32773ab67f51bcf7516489f183
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>