ubi: Fix wrongly integrated changes from lsk-v3.18-16.11-android

The commit <cc449563> ("Merge tag 'lsk-v3.18-16.11-android' into
'msm-3.18'")  wrongly integrated the changes associated with this
commit: <6cc9c7cf> ("UBI: Fastmap: Fix race after ubi_wl_get_peb()").

The culprit change mistakenly adds read lock ubi->fm_eba_sem in the
function ubi_eba_atomic_leb_change(). Fix it by removing the same.
Otherwise, it will result into deadlock in the other paths where it
wants to acquire write lock on ubi->fm_eba_sem.

Change-Id: I45f275284c6c4dc2fd35a4efb78c81986b64e040
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>