f2fs: fix to avoid mmap vs set_compress_option case

Compression option in inode should not be changed after they have
been used, however, it may happen in below race case:

Thread A				Thread B
- f2fs_ioc_set_compress_option
 - check f2fs_is_mmap_file()
 - check get_dirty_pages()
 - check F2FS_HAS_BLOCKS()
					- f2fs_file_mmap
					 - set_inode_flag(FI_MMAP_FILE)
					- fault
					 - do_page_mkwrite
					  - f2fs_vm_page_mkwrite
					  - f2fs_get_block_locked
					 - fault_dirty_shared_page
					  - set_page_dirty
 - update i_compress_algorithm
 - update i_log_cluster_size
 - update i_cluster_size

Avoid such race condition by covering f2fs_file_mmap() w/ i_sem lock,
meanwhile add mmap file check condition in f2fs_may_compress() as well.

Fixes: e1e8debe ("f2fs: add F2FS_IOC_SET_COMPRESS_OPTION ioctl")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>