ext4: remove unnecessary drop path references in mext_check_coverage()

According to Jan Kara's suggestion:
"The use in mext_check_coverage() can be actually removed
- get_ext_path() -> ext4_find_extent() takes care of dropping the references."
So remove unnecessary call ext4_ext_drop_refs() in mext_check_coverage().

Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220924021211.3831551-2-yebin10@huawei.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>