f2fs: don't get FREEZE lock in f2fs_evict_inode in frozen fs

Let's purge inode cache in order to avoid the below deadlock.

[freeze test]                         shrinkder
freeze_super
 - pwercpu_down_write(SB_FREEZE_FS)
                                       - super_cache_scan
                                         - down_read(&sb->s_umount)
                                           - prune_icache_sb
                                            - dispose_list
                                             - evict
                                              - f2fs_evict_inode
thaw_super
 - down_write(&sb->s_umount);
                                              - __percpu_down_read(SB_FREEZE_FS)

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>