vfs_fruit: detect empty resource forks in ad_convert()

vfs_fruit: detect empty resource forks in ad_convert()

For some reason the macOS client often writes AppleDouble files with a
non-zero sized resource fork, but the resource fork data is just
boilerplate data with the following string close to the start

  This resource fork intentionally left blank

A dump with apple_dump looks like this:

Entry ID   : 00000002 : Resource Fork
Offset     : 00000052 : 82
Length     : 0000011E : 286

-RAW DUMP--:  0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F : (ASCII)
00000000   : 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00 1E : ................
00000010   : 54 68 69 73 20 72 65 73 6F 75 72 63 65 20 66 6F : This resource fo
00000020   : 72 6B 20 69 6E 74 65 6E 74 69 6F 6E 61 6C 6C 79 : rk intentionally
00000030   : 20 6C 65 66 74 20 62 6C 61 6E 6B 20 20 20 00 00 :  left blank   ..
00000040   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000050   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000060   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000070   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000080   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000090   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000A0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000B0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000C0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000D0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000E0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
000000F0   : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 : ................
00000100   : 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00 1E : ................
00000110   : 00 00 00 00 00 00 00 00 00 1C 00 1E FF FF       : ..............

We can safely discard this Resource Fork data.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13642

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>