f2fs: call write_checkpoint under disabled gc

During the write_checkpoint, we should avoid f2fs_gc trigger to avoid any
filesystem consistency.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1046,7 +1046,9 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
 	cpc.trim_minlen = range->minlen >> sbi->log_blocksize;
 
 	/* do checkpoint to issue discard commands safely */
+	mutex_lock(&sbi->gc_mutex);
 	write_checkpoint(sbi, &cpc);
+	mutex_unlock(&sbi->gc_mutex);
 out:
 	range->len = cpc.trimmed << sbi->log_blocksize;
 	return 0;