btrfs-progs: tests: add case to verify large symbolic link handling in convert

The new test case will:

- Create a symbolic which contains a 4095 bytes sized target on ext4

- Convert the ext4 to btrfs

- Make sure we can still read the symbolic link
  For unpatched btrfs-convert, the resulted symbolic link will be rejected
  by kernel and fail.

Signed-off-by: Qu Wenruo <wqu@suse.com>

tests/convert-tests/027-large-symbol-link/test.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# Make sure btrfs-convert can handle a symbol link which is 4095 bytes large
+
+source "$TEST_TOP/common" || exit
+source "$TEST_TOP/common.convert" || exit
+
+setup_root_helper
+prepare_test_dev 1G
+check_global_prereq mkfs.ext4
+
+# This is at the symbolic link size limit (PATH_MAX includes the terminating NUL).
+link_target=$(printf "%0.sb" {1..4095})
+
+convert_test_prep_fs ext4 mke2fs -t ext4 -b 4096
+run_check $SUDO_HELPER ln -s "$link_target" "$TEST_MNT/symbol_link"
+run_check_umount_test_dev
+
+# For unpatched btrfs-convert, it will always append one byte to the
+# link target, causing above 4095 target to be 4096, exactly one sector,
+# resulting a regular file extent.
+convert_test_do_convert
+
+run_check_mount_test_dev
+# If the unpatched btrfs-convert created a regular extent, and the kernel is
+# newer enough, such readlink will be rejected by kernel.
+run_check $SUDO_HELPER readlink "$TEST_MNT/symbol_link"
+run_check_umount_test_dev