isisd: fix disabled flex-algo on race condition

isisd: fix disabled flex-algo on race condition

A particular flex-algo algorithm may remain in disabled state after
configuring it if its flex-algo definition is being spread in the area.

It happens sometimes that, in isis_sr_flex_algo_topo1 topotest, r3
flex-algo 203 is disabled on test8. It depends on the following
sequence on r3:
 1. a LSP containing the flex-algo 203 definition is received from
    either r1 or r2 (or both).
 2. the local LSP is rebuilt by lsp_build() because of the flex-algo 203
    configuration
 3. isis_run_spf() recomputes the algo 203 SPF tree

A 1. 2. 3. sequence results in a working test whereas 2. 1. 3. is not
working. The second case issue is because of an inconsistent flex-algo
definition state between the following:
 - in lsp_build(), isis_flex_algo_elected_supported_local_fad() returns
   false because no flex-algo definition is known.
 - in isis_run_spf(), isis_flex_algo_elected_supported() returns true
   because a flex-algo definition is found.

Set a flex-algo state lsp_build() depending on flex-algo definition
existence that is used later in isis_run_spf().

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>