dlm: add __CHECKER__ for false positives

This patch will adds #ifndef __CHECKER__ for false positives warnings
about an imbalance lock/unlock srcu handling. Which are shown by running
sparse checks:

fs/dlm/midcomms.c:1065:20: warning: context imbalance in 'dlm_midcomms_get_mhandle' - wrong count at exit

Using __CHECKER__ will tell sparse to ignore these sections.

Those imbalances are false positive because from upper layer it is
always required to call a function in sequence, e.g. if
dlm_midcomms_get_mhandle() is successful there must be a
dlm_midcomms_commit_mhandle() call afterwards.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>