mkfs.btrfs: fix symlink names writing

mkfs.btrfs: fix symlink names writing

Found by valgrind:
==8968== Use of uninitialised value of size 8
==8968==    at 0x41CE7D: crc32c_le (crc32c.c:98)
==8968==    by 0x40A1D0: csum_tree_block_size (disk-io.c:82)
==8968==    by 0x40A2D4: csum_tree_block (disk-io.c:105)
==8968==    by 0x40A7D6: write_tree_block (disk-io.c:241)
==8968==    by 0x40ACEE: __commit_transaction (disk-io.c:354)
==8968==    by 0x40AE9E: btrfs_commit_transaction (disk-io.c:385)
==8968==    by 0x42CF66: make_image (mkfs.c:1061)
==8968==    by 0x42DE63: main (mkfs.c:1410)
==8968==  Uninitialised value was created by a stack allocation
==8968==    at 0x42B5FB: add_inode_items (mkfs.c:493)

readlink(2) does not write '\0' for us, so make it manually.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>