btrfs-progs: fix len of read_extent_buffer for inline extent in restore

btrfs-progs: fix len of read_extent_buffer for inline extent in restore

Steps to reproduce:
	# mkfs.btrfs -f <dev>
	# mount -o compress-force=lzo <dev> <mnt>
	# for ((i=0;i<4000;i++)); do
	  	echo -n 'A' >> <mnt>/inline_data
	  done
	# umount <mnt>
	# valgrind --tool=memcheck --leak-check=full	\
	  btrfs restore <dev> <dest_dir>
output:
	==32118== Invalid read of size 1
	==32118==    at 0x4A0A4E4: memcpy@@GLIBC_2.14
	==32118==    by 0x43DC91: read_extent_buffer
	==32118==    by 0x421401: search_dir (cmds-restore.c:240)
	==32118==    by 0x422CBB: cmd_restore (cmds-restore.c:1317)
	==32118==    by 0x404709: main (btrfs.c:248)
	==32118==  Address 0x4c4f4ac is not stack'd, malloc'd or...

It is because when deal with inline extent, the read_extent_buffer
is now reading a len of @ram_bytes which is the len of the uncompressed
data. But actually here we want the len of the inline item.
So in the compressed situation, use the len of the inline item.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>