ext4: silence sparse check warning for function ext4_trim_extent

This fixes the following sparse warning:

     CHECK   fs/ext4/mballoc.c
   fs/ext4/mballoc.c:5019:9: warning: context imbalance in
   'ext4_trim_extent' - unexpected unlock

Signed-off-by: "Jon Ernst" <jonernst07@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>