exfat: fix error handling for FAT table operations

Fix three error handling issues in FAT table operations:

1. Fix exfat_update_bh() to properly return errors from sync_dirty_buffer
2. Fix exfat_end_bh() to properly return errors from exfat_update_bh()
   and exfat_mirror_bh()
3. Fix ignored return values from exfat_chain_cont_cluster() in inode.c
   and namei.c

These fixes ensure that FAT table write errors are properly propagated
to the caller instead of being silently ignored.

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>