ext4: remove unnecessary lock/unlock of i_block_reservation_lock

This is a leftover of commit 71d4f7d0

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>