ext4: remove repeat assignment to ac_f_ex

Call trace to assign ac_f_ex:
ext4_mb_use_best_found
	ac->ac_f_ex = ac->ac_b_ex;
	ext4_mb_new_preallocation
		ext4_mb_new_group_pa
			ac->ac_f_ex = ac->ac_b_ex;
		ext4_mb_new_inode_pa
			ac->ac_f_ex = ac->ac_b_ex;

Actually allocated blocks is already stored in ac_f_ex in
ext4_mb_use_best_found, so there is no need to assign ac_f_ex
in ext4_mb_new_group_pa and ext4_mb_new_inode_pa.
Just remove repeat assignment to ac_f_ex in ext4_mb_new_group_pa
and ext4_mb_new_inode_pa.
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-19-shikemeng@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -4636,10 +4636,6 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
 		BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
 	}
 
-	/* preallocation can change ac_b_ex, thus we store actually
-	 * allocated blocks for history */
-	ac->ac_f_ex = ac->ac_b_ex;
-
 	pa->pa_lstart = ac->ac_b_ex.fe_logical;
 	pa->pa_pstart = ext4_grp_offs_to_block(sb, &ac->ac_b_ex);
 	pa->pa_len = ac->ac_b_ex.fe_len;
@@ -4690,10 +4686,6 @@ ext4_mb_new_group_pa(struct ext4_allocation_context *ac)
 
 	pa = ac->ac_pa;
 
-	/* preallocation can change ac_b_ex, thus we store actually
-	 * allocated blocks for history */
-	ac->ac_f_ex = ac->ac_b_ex;
-
 	pa->pa_pstart = ext4_grp_offs_to_block(sb, &ac->ac_b_ex);
 	pa->pa_lstart = pa->pa_pstart;
 	pa->pa_len = ac->ac_b_ex.fe_len;