erofs: better erofs symlink stuffs

erofs: better erofs symlink stuffs

Fix as Christoph suggested [1] [2], "remove is_inode_fast_symlink
and just opencode it in the few places using it"

and
"Please just set the ops directly instead of obsfucating that in
a single caller, single line inline function.  And please set it
instead of the normal symlink iops in the same place where you
also set those."

[1] https://lore.kernel.org/r/20190830163910[email protected]/
[2] https://lore.kernel.org/r/20190829102426[email protected]/
Reported-by: Christoph Hellwig <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>