dma-buf: fix sleep-while-atomic in dmabuffs_dname

There is a sleep-while-atomic reported when the selinux tries to
dump the file path name of a dmabuf file using d_path(). Fix it.
Flow will be like below:
flush_unauthorized_files -->
  iterate_fd -->
    spin_lock --> Start of the atomic section
      match_file -->
        file_has_perm -->
          avc_has_perm -->
            avc_audit -->
              slow_avc_audit -->
	        common_lsm_audit -->
		  dump_common_audit_data -->
		    audit_log_d_path -->
		      d_path -->
                        dmabuffs_dname -->
                          mutex_lock--> Sleep while atomic.

Change-Id: Ieed88a9093355d160e1c194d31fe33f1c7dadf60
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>