dm: fix some sparse warnings and whitespace in dax methods

Eliminate these sparse warnings:
drivers/md/dm.c:1062:9: warning: context imbalance in 'dm_dax_direct_access' - unexpected unlock
drivers/md/dm.c:1086:9: warning: context imbalance in 'dm_dax_copy_from_iter' - unexpected unlock

Signed-off-by: Mike Snitzer <snitzer@redhat.com>