What does fs:0x30 provide in Linux?

The fs segment register is used in x86-64 Linux to point to thread-local storage. See How are the fs/gs registers used in Linux AMD64? So this instruction will xor the rdx register with the value found at offset 0x30 in the thread-local storage block.

This code is part of a pointer encryption mechanism in glibc to help harden against certain exploits. There is some explanation of it at https://sourceware.org/glibc/wiki/PointerEncryption. The value at fs:0x30 is an "key" for a trivial "encryption" algorithm; pointers are xor'ed with this value (and then rotated) when they are stored, and rotated back and xor'ed again when they are retrieved from memory, which recovers the original pointer.

There is no particular significance to the number 0x30; it just happens to be the offset where that value is stored. You can see in the inline assembly that this number comes from offsetof (tcbhead_t, pointer_guard); so the storage at the fs base address is laid out as a tcbhead_t struct, and given the other members that it contains, the pointer_guard member has ended up at offset 0x30. So looking at the name pointer_guard for the member is more informative than its numerical offset.