objtool: Extract code to validate instruction from the validate branch loop

objtool: Extract code to validate instruction from the validate branch loop

The code to validate a branch loops through all instructions of the
branch and validate each instruction. Move the code to validate an
instruction to a separated function.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://patch.msgid.link/20251121095340.464045-9-alexandre.chartre@oracle.com